We went to Elveden last Spring and despite what I'm about to write, we had a great time and would go again.
If you can afford the better accommodation, like Lomond's which sounds like the Executive Lodge (is that what it's called?) and also find the newer accommodation, then do it. Some of the accommodation is really run down and the basic villas really are very basic. We had a mid range villa and it was just about acceptable. The posh detached ones with hot tubs, etc looked so much better. If you've got taste in decor, you'll just have to leave that at home as their idea of stylish decor is a bit different to anyone else's.
I have to say I do understand the 'motorway service station' comment. It's pretty lacking in atmosphere and the architecture is pretty mundane, not as much effort as Disney say but you have to take it for what it is.
You need to prebook your activities the minute you get the timetable and booking info as everyone else who is going the same week as you will have got theirs and be booking like mad. Book the spa treatments first if you can, there wasn't availability for even as much a manicure when I tried to book! You can cancel things and get a refund when you get there so if in doubt, book it.
I thought the food was pretty disgusting and totally overpriced but at the same time, unless you've prebooked it was quite hard to get into most restaurants. They have a captive audience and no competition so they get away with murder, imo. I wasn't very impressed with the supermarket (fruit = apples, bananas and oranges) but Elveden has a fantastic farm shop next door which was a lifesaver and we decided to do our own cooking after the first couple of days. Might be something similar around Longleat?
I found the lunch arrangements very strange with hardly any restaurants open. At Elveden only one place was open but it closed pretty early, at 1.30pm or 2pm, and only seemed to serve microwaved potatoes with baked beans and burgers. I would have welcomed a sandwich or salad place and if they do bring big brands in, something like Pret a Manger would be ideal. I came to the conclusion that most people made their own lunches.
It is expensive and you need to be really organised to make the most of it but the pools are free and there are lots of nice play areas, most of which will keep dcs of 3 happy without too many extra activities required. Keep your fingers crossed for good weather as that makes such a difference - we had good weather every day. The creche was very good with great activities for our DD of 3 but this is an additional charge.
After all that whingeing, we really did have a good time. Living in a city, we really felt like we were right out in the country. The dcs saw squirrels and rabbits outside the villa and cycling through the trees each morning with no cars and the dcs in a Burley trailer behind was lovely.
Google for lots of specific reviews for each location - I got loads of information online before booking. I think you need to use different spellings - Center Parcs and Centre Parcs. I've seen various comments and I think Longleat is the one that's hilly which can be an issue for pushchairs.
Hope you do go and that you enjoy it.