Being UK based I would need Customs, avgas, and IFR to be sure of getting back out even if the inbound wx is fine.
That basically means Ruzyne LKPR (€150 or so) or Vodochody LKVO (much less).
Kbely would be good but last time I looked to go there the data said it is open only to visitors to the based factory, or turboprops and above, though one chap who used to rent a piston PA46 reported that he got in (or could get in) with that.
Letnany is grass, no Customs, no IFR.
Prague is an exceptional location for a meet-up. I am not biased of course 
Is the idea to meet in town or at an airfield?
If airfield, then it will be difficult. Vodochody is at the end of the world, not really a place to meet up. Letnany is nice but no customs of course. Ruzyne is too big and too expensive and not interesting for a meet up either.
For meeting at an airfield, Karlovy Vary LKKV would be good. A very nice town and it ticks all the boxes.
Up for that... guessing that it'd be an overnight... so as long as there is enough time to do the paperwork to rent a spamcan overnight - no issues!
Plus I would probably need AVGAS and would be VFR at the moment; but wouldn't need customs because I can clear at EDDS in the A320 British Airways always lend me to fly home in... I wish :)
In the city, of course 
If you are going to meet up at an airport cafe, then there is a million choices, though at some of them you may have to make do with a cucumber which was sliced the day before...
Customs can be arranged in Letnany with 24h PNR. I am not sure about the price but would expect up to 40 EUR...
I cannot recommend Ruzyne, last time I was there (which is years ago) it was not only expensive, but also quite inefficient.
I've heard Letnany is a bit bumpy, due to animals digging holes in the ground...
I've been to Ruzyne a few times and it was always very efficient, but why pay the money if Vodochody is just down the road?
This needs to be checked but at Ruzyne you could just file the flight plan and fly there, H24, but Vodochody isn't H24 and Customs are PNR.
As I wrote in the other thread my experience with Vodochody was OK - the only possible issue is that you have to be precise with your arrival and departure due to customs and border police arangement. However, beeing on precise schedule with meetings I attended, it wasn't problem for me.
A friend of mine has just been to Vodochody two weeks ago with some friends in 2 C-172s, and as far as I heard they all LOVE it. I can get more info if necessary.
go for LKVO, forget about LKPR ;-) if you are jus looking for destination and you want to get further to east, think of LKKU, home of Aircraft Industries (ex Let) and Evektor companies. Private field, but customs needs to be arranged with 24 hours notice. Staying overnight might give you a chance to taste local wine - just make sure you are not planning morning departure.
