I have a pair of size 18 M&S ones in very good condition which I'm happy to donate. Sadly, they're under the bump and they do slide down but it was suggested that wearing braces might help . I'm getting towards the end of my pregnancy and was planning to charity shop them but I'm happy to send them to you if you're interested. (I won't be offended if you're not as the under the bump ones really are a pita.)
I have some size 18 next over the bump ones that I find really comfy but being quite chunky-thighed found I needed to go up to 18 from my usual 16, not sure if they do a 20 or not - you have to set up an online credit account which is a bit of a faff but then ordering and doing returns is really easy. I think they were £26 - not super cheap but I have literally been wearing them every day except when they're in the wash, and I'm about to pop now.
I wear braces with mine And they're over the bump ones from Jojo maman bebe. I live in them, and their sizing appears to be quite generous. I was a pear shape 21 weeks ago, and 2.5 stone later (and twins) am quite broad in the behind. Again, they weren't cheap - about £35, but I'm living in them. So much so I think it's worth investing in another pair.
What leg length are you after? I've got some LaRedoute over the bump jeans that I've shortened to 27" that you're welcome to have. There's a bit of extra fabric in the hem to lengthen them if needed. They are a 20 and both the waistband and the over-bump panel have separate adjustable elastic-y bits. PM me if you're interested.
Urm I'm not sure what size Mamas and Papas go up to but the maternity jeans I've bought from there are elasticated on the front (under or over bump) but normal jean-waist around the back so they don't go anywhere.
THey have a sale on atm, I popped in yesterday and got some for £12 reduced was £42.
Asda! Have some great black ones. Also, they do some fab black yoga pants with a stretchy panel and I have lived in them for the last 2 weeks. Literally. I smell.
Also consider legging and long tops?? Boden have some great leggings which are almost worth the cost - they come up quite large so go for a size below. They wash well and are super comfy.
My H and M jeans are in a size 22 I think - I am normally a 16-18 - and barely fit me now, thigh-wise. They clearly do not recognise that well known condition known as total body pregnancy and think that the only thing that gets biger is people's tummies.
Would also suggest Next - when I was pregnant I had some H&M maternity jeans but they didn't last me the whole pregnancy, so got a replacement pair from Next (over-bump) and they lasted right up until the last day. They were very generously cut, comfortable and reasonably priced.