In the process of booking train tickets at present from Kings Cross to home.
(Got a free one from East Coast which is an advance ticket so that train only - back-up option is to book a cheap seat on the next train / hour later)

Going to be flying into T5 on the A380 (in WT at present but may upgrade to WT+ - nothing going in CW MFU wise) and this arrives at 3:55pm.

I'm assuming immigration will be a little bit slower with a larger aircraft but is this a bad time of day for queues due to other planes arriving or would most of the larger aircraft be early a.m. arrivals?
Hand luggage only.

HEX + tube will take 45-60 minutes and the tube a few minutes more so there's not much of a difference here.