We will be taking trains from Portsmouth to Weymouth, Exeter, St. Austell, Penzance and St. Ives Plymouth, London . Are they on time and will they be full or should I make a reservation? This trip will take place between late March and mid April. Thanks.
Trains usually run on time, but delays are possible. Some of your trains won’t allow reservations, while others will.
A key element for this type of travel is that everyone must be able to carry own bags up and down a flight of stairs. If this is hard for you, you may find train travel uncomfortable.
Whether or not you make a reservation is up to you.
Whether or not the trains are late varies depending on the day and what is going on that day.
As for reservations, I toured in the area in May a few years ago and a few trains were standing room only.
Tickets may be significantly cheaper if booked in advance.
There is no train between Weymouth and Exeter- you can route via Bristol (Weymouth to Bristol is a very pretty route) but I recommend taking the Breezer bus for £3 along the western part of the Jurassic Coast via Bridport and Lyme Regis. That takes you to Axminster station for hourly trains to Exeter.
As you say 'we' you need to buy a Two Together Railcard to give you 1/3 off rail fares, which costs £30- https://www.twotogether-railcard.co.uk/
To avoid booking lots of separate tickets and getting caught up in the "let's find the cheapest fare and be tied to specific trains" game I suggest buying a Freedom of the South West Rail Rover Ticket for £180 each 8 days in 15 (with the railcard that reduces to £119.85).
So your £30 railcard saves you £60 each.
That covers all your trains from Portsmouth all the way to Penzance and back towards London as far as Swindon. A train from Penzance normally routes via Westbury (not Swindon) but whatever just buy a normal add on ticket from Swindon or Taunton (last stop on the West Country trains before Reading) to London.
At £15 a day that gives total flexibility after 9am weekdays (any time weekends) and is very good value. Valid on all operators trains in the area. You can make reservations (where applicable) either in person at rail stations or on line on the GWR website/app.