I spent two nights in Poole in 2017 because I planned to arrive in England by ferry from Jersey. It didn't work out that way (3 days in a row of ferry cancellations in the summer--beware), but by the time I realized I needed to fly instead, it was too late to get a refund on the room in Poole. (It was foolish of me not to go ahead and cancel anyway, without a refund, so I didn't have to scramble to get from Luton Airport to Poole, but under pressure one doesn't always think clearly, or at least I do not.)
Poole isn't an awful place, or unsafe, but it seemed to be a down-market beach town. I will be interested to see what other people think, but there are very, very few places in Europe that I wouldn't want to revisit, and Poole is one of them.
There are other places along the coast that are much nicer, I think. I'm not the best person to suggest an alternative, having spent very little time in the area, but I thought Weymouth was fine, though nothing remarkable. While there I went to the Abbotsbury Sub-tropical Gardens and Portland Island (art in situ). Both those trips involved considerable walking but were worth it to me.
I stopped briefly in Bournemouth to see the Russell-Cotes Art Gallery and Museum [name corrected]. Bournemouth was attractive. Lyme Regis looked better than Poole when I passed through it on a bus, but I didn't see much of it.
If you do end up in Poole, one thing I can recommend is the Compton Acres Gardens. They were very nice.
Moving west, I enjoyed the one night each I was forced to spend in Exeter and Truro due to not having booked lodgings far enough ahead of time to get something affordable on the coast. They both seemed to be nice, real-world towns with some interesting historic architecture.
My coastal stays were in Mevagissey (convenient to The Eden Project and the Lost Gardens of Heligan) and St. Ives (very touristy, but a good place for lovers of ceramics and art). Anywhere coastal in Cornwall requires locking down reservations as soon as you can. And you need to book for dinner, too, unless you want to consume food purchased from a grocery store or a fish-and-chips truck. Believe me, it's not just fancy restaurants that get booked up.
It sounds as if you plan to do the entire trip by public transportation, as I did. It's workable but will be slow in Cornwall, especially, because the buses don't follow the coastline (probably the roads don't, either). A lot of the buses I used were double-deckers, which offered a great view of the countryside from the upper deck, because you could see over the hedges lining so many of the roads.
I'll post again with some ideas for Jersey. I have a couple of errands to run right now.
