In a Cold War gone hot scenario London won't be there for the Soviets to occupy and indeed neither will Moscow for that matter.
The first link manages to mention two myths in it - firstly Hollywood style EMP and secondly the British Strategic St**m R***rve (as a member of Sub-Brit I have to write it like that

). It also contains a ASB scenario for a Soviet invasion of the UK, so well done to the bloke that wrote it.

To disect the scenario the Soviets disable British defences using EMP (which wouldn't work because military electronics are protected against it) and then the Brits race them to Edinburgh in a loco that doesn't exist from a stockpile that doesn't exist.
If the St**m R***rve existed we would have either found the locos by now, or the documents mentioning it would have come to light. Hundreds of steam locos just can't vanish into thin air and leave no trace.

In any case the diesel locos designed and built in the '50s and '60s, such as the 20s, 37s and 47s, would work well enough post nuclear strike.