Well, lets break down the problem.
1. Can you fly night VFR in Belgian airspace? If so, then you can get to the London FIR boundary.
2. Can you fly under IFR in VMC in a Belgian aircraft, with a Belgian PPL + Night qualification? (The UK part of this is pretty much irrelevant, as the UK allows this, but does the Belgian leglisation put restrictions on this worldwide?)
If the answer to this part is yes, then you're there.
If the answer to this part is no, then there is no what that I can see that you can do it. (You can't in an Irish registered aircraft by the way. You need an IR to fly under IFR, no matter where you are.)
SVFR is only available in a control zone (not all class A, only a control zone CTR). As there is no control zone that links the edge of the UK FIR to Biggin Hill, it's not possible to go entirely SVFR in UK airspace.
Whether anyone would be bothered is another question entirely though. I suspect, but only suspect, that the answer is that noone would be bothered, unless you had an incident.
dp