I think Tom has it spot on there. The larger summits are generally ok if there are enough people around to hear you (VHF is slightly dead in the UK). A lot of the single point summits may struggle due to other larger surrounding hills, especially further north
My personal experience:-
Southern regions (SE/SC/DC) are really hit and miss on VHF. The take off is generally ok but I’ve struggled many times to get the required 4 contacts. I’ve failed almost as many activations as I’ve succeeded using VHF alone. As other people have said, it depends on the day. Weekends probably stand a greater chance of success.
North & South Pennines & the Lake District have a bit of a VHF following, but remember that other hills may block your signal, so there are summits where VHF just won’t work.
I seem to recall that North Wales wasn’t too bad on VHF (although it’s been several years since I’ve been there).
South Wales seems to have a bit of a VHF chaser following and will generally succeed on VHF. That said, I once did a joint activation of Pen Y Fan (GW/SW-001) with a couple of friends. As there were three of us to carry the kit, we decided to put on a bit of a show and went all out with a fairly large mast, a beam, a suitably large colinear, batteries and an amplifier.
Our signal was definitely getting out as we made two contacts just outside London, one in Epsom & one in Watford (both around 140-150 miles as the crow flies). Then nothing. after around an hour of swinging the beam in all directions whilst calling CQ (on both the beam & the colinear), we had zero replies. We gave up and called the activation a failure!!!
I can’t speak for the other regions as the regions mentioned above are the only regions that I’ve been to.
As people have said, it is possible to qualify most summits on 2m but HF is a more reliable bet if you want to guarantee qualifying the summit.
I’ve used 20m and there are a couple of American stations and a Canadian which often call in.
40m seems to be a favourite for a lot of other European SOTA activators & chasers.
In terms of getting local stations, there is a bit of an HF following across the UK on 80m & 60m. When I’m chasing on HF there always seem to be a few familiar callsigns that pop up.
80m & 60m is probably your best bet, although be careful on 60m as there are additional restrictions which make this band a little confusing to say the least (the RSGB has recommended spot frequencies on it’s website).
My personal view would be to take the 817 (a very versatile rig that can do virtually anything) with antennas for both 2m VHF and an antenna for either 80m or 60m.
60m very popular and is a lot quieter than 80m (much less QRM from other stations). 60m is a good choice if the conditions are right.