Oops, quite right. Harbour cities starting with P. Difficult to remember...FTFY (sorry!)
There's bike racks on the sleeper in the luggage compartment in the same car as the seats (unless they use different train sets on the lowland sleeper these days). I used it last year on my trip to the highlands.I looked at this, but the bike+sleeper option requires bagging the bike up and it possibly getting couriered, and I'll have a ton of luggage.
Indeed, if you have a late night ferry that does sound like the best option. Shouldn't be too difficult a navigation across the big smoke.Easiest would be a Virgin train to Euston, then hoof across to Waterloo and get the SW train to Portsmouth and ideally straight onto the overnight ferry.