I have one to add to this, https://www.unbeatablehire.com/ . They have the same sort of reviews, I was quoted £459 for a week but they apparently add on lots of hidden extras and charge your card for them without permission. Also seems that the campervans are not in good condition.
I used https://easicampervanhire.com/glastonbury-festival-camper-van-hire/ last year, they were great. Campervan was pretty much brand new, water butt included and filled before we picked it up along with all the mod cons. Only £1100 for the week, which we split between 4.