There is confusion between "London 124.6" and e.g. "London 130.175".
The first is London Information and the second is London Control.
So it is context sensitive. VFR traffic will only get the first one, and they do not officially have radar and certainly are not allowed to say anything suggesting they can see you in any way. IFR traffic should get the 2nd one (the frequency varies) and if you get the 1st one then you have been dumped out of CAS and your enroute IFR clearance has been cancelled. This happens less often nowadays.
The locals will know that 124.60 is London Info (the regional FIS service).
It's not a great system...
In Eurocontrol IFR flight, you don't normally report your level on a handover unless you are still climbing or descending. The callsign should be enough; all the enroute IFR controllers have you on their screen (your type, filed route, etc).
For a one-off or occasional visit, the UK is too complicated.
Can you be specific on what exactly is complicated?
The airspace isn't, AFAICS. It is mostly G, and the 1:500k CAA VFR chart does not have ambiguities in the way airspace is labelled. And in G you can just fly around...
In Eurocontrol IFR flight, you don't normally report your level on a handover unless you are still climbing or descending.
In fact, I have never heard people doing so...;-)
the 1:500k CAA VFR chart does not have ambiguities in the way airspace is labelled.
It may all seem clear to you, because you are used to it. For a german, it is a nightmare with all those TMA sectors, CTAs, AWYs, ATZs, MATZs and AIAAs...
I have never heard people doing so
Doing what?
Reporting altitude when handed over to the next controller inside CAS. Everybody does it.
Exactly, it is standard IFR practise to always report it together with your waypoint. However, in the UK I was constantly asked by radar (not FIS) to "contact Scottish Control 123.45 and report your altitude", it was on almost every handover. This seemed weird to me because first of all I think I have to read it back like this (it is an instruction after all) and 2nd they should know my altitude and I would do it anyway because it's IFR standard.
Can you be specific on what exactly is complicated?
The airspace and the fact that Jeppesen have stopped doing VFR maps for it, the lack of proper FIS with traffic information, the terminology, the strange way of joining patterns, the ubiquitous PPR requirement, the fact I have to spend 3 € on a hi-vis jacket... 
I am audacious, no problem, I just noticed that the UK has more idiosyncrasies as any other country I've ever flown in. I can understand that the US are a bit different because they are huge and self-sufficient and generally think they invented the world but the UK are part of the EU. I'm hoping that the EU will unify airspace layout and flying in general so we also get some advantages of EU regulation not just disadvantages.
PS: "What type of service do you request?" "Aehm, well, let me check my list... deconfliction, yes, deconfliction!" "Only basic service possible, do you request basic service?". I would have expected this kind of bizarre dialog in Africa but it was somewhere else 
it is standard IFR practise to always report it together with your waypoint
It's not normal on a straight handover to the next sector frequency.
Also boscomantico said
I have never heard people doing so
Even in Class G, when handed over from one radar unit to the next, you don't need to do it.
There is a difference however between being asked to "freecall XYZ" and being asked to "contact XYZ". The latter means they have your details and the handover has been coordinated. I suspect that is another UK peculiarity.
However, in the UK I was constantly asked by radar (not FIS) to "contact Scottish Control 123.45 and report your altitude", it was on almost every handover.
Possibly because you were OCAS?
London Control doesn't do that, but then LC offers a service only to Eurocontrol IFR traffic in CAS.
I can understand that the US are a bit different because they are huge and self-sufficient and generally think they invented the world
I resemble that remark.:)
Possibly because you were OCAS?
London Control doesn't do that, but then LC offers a service only to Eurocontrol IFR traffic in CAS.
This was on the inward flight where I didn't dare to go "off road" in G. The instruction was exactly like I said and this is also what all the airliners got.
It's not normal on a straight handover to the next sector frequency.
Maybe it's a German thing but I was taught during training that an IFR call to a new station always goes like this "London Control, D-EXXX, flight level 150 inbound Clacton VOR". I have to pay more attention during my next flight but I think this is what every airliner does, too, sometimes abbreviated. After takeoff, I do the same when transitioning from tower to radar "Langen radar, D-EXXX, climbing flight level 150 passing 3000 on Dinkelsbühl 4 bravo". I thought that was ICAO.
Maybe it's a German thing ...
No, all over ICAO and even in the Unites States across the pond (I quote from FAR 91.183: "If you have been handed off from another facility, your call should begin 1) Name of facility, 2) Your full aircraft identification, and 3 Your altitude (s) as level, climbing to, descending to, as well as any ATC speed restrictions.)"
But as usual, there are some exceptions. Especially in the London area, many ATC sectors are supposed to be called with callsign only. This will be stated either in some difficult to find place in the Jeppesen introductory part or somewhere on the approach plate or in the 10-1 section of an aerodrome. Therefore the instruction "Call Heathrow approach on 123.45 with your altitude" might make sense when Heathrow approach would usually be called with callsign only.
The PPR requirement means that all non-based pilots can be advised about noise abatement.
Most of the airfields I fly from are surrounded by extremely noise-phobic locals.
