Can you post a section of the chart?
It’s on your latest IFR chart:
OCK to BPK at 3,500ft; BIG to HEMEL at 3,500ft; BIG and CPT at 5,500ft; LAM to OCK at 5,500ft
Then if you look at your latest VFR chart the CAS base for the LTMA there is mainly 2,500ft.
OCK to BPK is M185, MEA is 9000ft
BIG to CPT is L9, MEA is 9000ft
LAM to OCK is L980, MEA is 10000
You have an out of date chart! 
Yeah, right….
But, seriously, I don’t work for NATS and I have never read the SRD, but I think what is loosely called “GA crossings” are not actually in the SRD and are not on these airway routes. Practically all traffic crossing the LTMA is radar vectored, and it is only if the ATCO’s vector would just happen to take you to say CPT that he will give you a DCT CPT.
If the LTMA had to be operated procedurally (no radar) the system would collapse. So they have multiple radar backups, which are very much “top secret stuff” for obvious reasons.
As I said, ATCOs posting in various places have reported that there are two GA crossing routes which run N-S and they are at FL090/100, and I have no idea where they are because I have been doing this since 2006 and have never had the same route twice. Also it would be only by accident I would be at FL090/100 and clearly one can go higher. But it is possible that the system maintains those two levels free, for GA crossings, just in case somebody turns up.
Achim knows the SRD I am sure
The other thing is that no matter what, you can’t fly some of these crossings in a SEP because you would be out of glide range over a congested area. Reportedly (this is unverified) the UK CAA has declared the Thames river as a congested area so a glide into it is not a legit option. IME, ATC won’t give these routes to a SEP ever, even at higher levels like FL150 where one would be OK to glide clear.
East west you can get more direct routings but north south seems very unlikely. And it is interesting because they will vector you over London coming back in from Europe down to FL100-080 as part of a descent profile so it seems the rule is more about climbing up over the LTMA or trying to get a low level flight across London.
EG ENR 3.1 LOWER ATS ROUTES
Q3 OCK-HEMEL: lower limit FL175, minimum IFR cruising level level FL190
T420 BIG-HEMEL: lower limit 3500 ft, minimum IFR cruising level level FL120
M185 OCK-BPK lower limit 3500 ft, minimum IFR cruising level level FL90
etc.
That is an astonishing way to write something that is clearly unworkable
I am still looking for an “IFR chart” which shows those routes.
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T420 BIG-HEMEL: lower limit 3500 ft, minimum IFR cruising level level FL120
The minimum level eurocontrol has for T420 on BIG-FINCH-HEMEL is F110 (well actually F105). I wonder where this discrepancy comes from.
