[Rainbow Six]

I doubt if Scotland Yard would have survived. It's the HQ of the Metropolitan Police and as such is an actual location rather than an organisation. It's located in central London, a short walk from Parliament, so would almost certainly have been destroyed when London was nuked.

What you are likely to have surviving is Special Branch (often known as "The Branch"). Whilst the Met's is the largest, each British police force has its own Special Branch. Special Branch is responsible for most anti terror work* and works closely with the Security Service (MI5). (This may have changed in the last few years, but at one point MI5 had no formal powers to arrest anyone, so if MI5 were carrying out an operation where an arrest was likely Special Branch officers would accompany them).

All Special Branch officers are routinely armed; their reputation is such that you don't want to mess with the Branch...if a Special Branch officer came to my house I'd be worried...

There are also Special Branch officers based at major ports and airports, where they liaise with the Immigration authorities.

So there would probably be a number of Special Branch officers operating in the UK at least; I'd imagine they would probably supply "muscle" to MI5 operations as and when required. a lot of these operations would probably be black ops types against the various separatist and marauder groups operating in the UK.

* Above is based on the T2K timeline; I believe there has been some changes to the anti terror structure since 9/11 with special departments being formed for CT work.