If you want my advice, I'd recommend the following:
1. Get a synthetic (rather than down) sleeping bag: far cheaper, you're not worried about it packing down really small and if it gets damp, it'll still keep you warm(ish). It also suffers less being kept in its stuffsack long-term.
2. Get a sleeping mat which will raise you off the floor with a decent "R value"... sleeping bags keep you warm because of the (warm) air trapped between the fibres inside the filling... when you lie on a sleeping bag, you compress the underside meaning it has limited insulation value compared with the top half. You will be more likely to get cold if you lie directly on the van floor, even if it is carpeted. I'd actually say this is just as important as the sleeping bag for a good night's sleep and not just for "softness" but warmth. I use an Exped SynMat 7 UL but that is pricey for your budget and there are cheaper options which don't cost the earth... again, you don't need small pack size or low weight so don't pay for it.
3. Ignore limit/extreme temperature ratings. Having said that, my "go to" bag has a comfort temperature of 2 degrees and I've used it happily in snow-bound winter camping in a tent (def colder than that), albeit with a down jacket and base layer on.
4. Don't forget you can wear more clothes and buy liners if you decide to go out in colder weather than your bag is designed for. I take a 3 season bag in the winter because of weight/pack size but wear warm clothes. I've never been cold, but I realise not everyone likes to do this.
So I'd suggest spending about £30 on a sleeping mat (or inflatable) such as the Thermarest Ridgerest Solar - assuming you can find some space on the floor to actually put it
Then spend about £40-50 on a synthetic 3-4 season sleeping bag - something like a Vango Nitestar 450 which is heavy, doesn't pack down and is synthetic... none of which are features you need to worry about (or pay for unnecessarily)... or get a cheaper (less insulating) bag and a liner to use in cold conditions.
A mummy bag is more confined (you've said you're not a fan of this I think) but they are warmer, especially for your feet which can get cold, even with warm socks on.