South coast itinerary and base

Being as you have to go to Manchester, why travel all the way down to the south coast when you have loads of places you could visit that are not so far from Manchester? Consider north Wales calling in at Chester on the way for maybe 1 night?http://www.visitwales.com/explore
http://www.visitchester.com
http://www.conwy.com
https://www.nationaltrust.org.uk/bodnant-garden
http://cadw.gov.wales/daysout/caernarfon-castle/?lang=en
https://www.snowdonrailway.co.uk

www.nationalrail.co.uk
www.virgintrains.co.uk

I agree with the previous post, but if you really want to spend those days in Cornwall then rent a car and have at it. You can get to Cornwall by train, and within Cornwall there is a train station not far from St. Michael's Mount, but to really get around and enjoy various points of interest you're much better off with a car.

I wouldn't try to fit Portsmouth in, much less Dover. If you only have 4 nights on the front part of the trip and Cornwall is your priority, spend the time in Cornwall.

Driving from Cornwall to Manchester will take you most of a full day, so factor that in. Be aware that driving on secondary roads takes more time than it looks like on a map.

My last drive from Cornwall (Plymouth actually as I was dropping someone off) to Portsmouth took just over 6 hours mainly due to roadworks, I wouldn't want to waste precious time doing that so I would stick to Cornwall.

Brighton might suit you. ( Full disclosure, I live their so might be biased!)

It has one of England's oddest palaces, the Royal Pavilion.

https://en.m.wikipedia.org/wiki/Royal_Pavilion

It also has a nice seafront and pier.

From there you can easily (15 minutes by train) reach Lewes, a historical small town slightly inland. It has a castle and a brewery plus some nice local pubs.

Further afield you can get to Arundel Castle in one direction

http://www.arundelcastle.org/

And Battle Abbey in the other

http://www.english-heritage.org.uk/visit/places/1066-battle-of-hastings-abbey-and-battlefield/history-and-stories/

Brighton is only an hour from London by train.

Alan

Depends what you want to see. You could do all the places in your post, but you're describing a road trip not a series of visits from a base. If you like a road trip then a train to Cornwall and heading east along the coast visiting various places on the way could be fun. West of Portsmouth is more scenic and picturesque. East of Portsmouth is over developed and not very pretty on the coast, but has interesting exceptions such as Brighton, and beautiful countryside and towns just inland (Chichester, Arundel or Lewes for example).

Portsmouth historic dockyards are fascinating for history buffs (brand new Mary Rose museum well worth a visit) as is Dover Castle. I'm afraid that the attractions for each town are somewhat more limited though. Portsmouth got pretty smashed in the war although there are some nice remnants, and Dover is a workaday port town really.

If a road trip isn't what you're after then maybe consider focusing on the SW part - Devon and Cornwall and maybe up as far as Dorset. St Michaels Mount, St Ives, Tintagel with King Arthur's Castle, the tin mining history and spectacular beaches and coastline and pretty villages will be plenty for a few days rxploring.

I second Alan's Brighton suggestion. It is an interesting mix of Regency, 1950s seaside, hippie and hipster. Enough in Brighton/Hove for a couple of days. Lots of interesting places nearby: Lewes, Tunbridge Wells, Hastings, Arundel, Pooh Sticks Bridge (likely not accessible by train), etc.

I lived on Shoreham and then Hove for a few months. I quite enjoyed wandering the Lanes, the Boardwalk and the Royal Pavillion.

Having taken the train to Cornwall previously, I recall it took the better part of a day to get there. Perhaps save for another trip.