It boils down to your cruising level. For instance if you leave Ostend and climb to FL60 IFR, as soon as you cross the UK FIR boundary you leave controlled airspace (base of CAS there is FL85 IIRC). Ostend has no other option but to pass your details to London info.
London info will try to arrange a join when you hit controlled airspace again west of DVR, but as mentioned it's generally too complicated for the LTMA to accommodate this.
If you were to cruise at FL100 then it would be a totally different matter. Brussels control would coordinate you with the relevant London sector and you would remain inside controlled airspace.
Originally Posted by
custardpsc
My suggestion was to refuse the handover and request handover to london control instead, after all you are in or about to be in their airspace... ].
Being handed over to London info means you are going to be outside controlled airspace on crossing the FIR boundary. As mentioned London info can not work traffic inside controlled airspace. Refusing to be handed over would likely result in being told to squawk 7000 and continue en-route.