They would certainly make a go for London, for obvious reasons.
I reckon that Newcastle and Edinburgh would be more likely targets than Manchester and Liverpool. Both are large industrial centres and one's a capital city; the big draw that they have for the Russians - as opposed to Manchester and Liverpool - is that they're on the East coast. Any 1949 nuclear attack is going to be conducted by slow, heavy bombers coming from bases in Eastern Europe or Northern Russia.
Given how scant the USSR's bombs are in this scenario, I imagine they'd rather attempt to hit cities that turn up right on landfall rather than have to cross 200 or so miles of enemy territory (all with observers, AAA, fighters etc) and back again. This is why I reckon Newcastle (much as I hate to say it) and Edinburgh are more likely targets.