By isofix you mean you want an isofix base to clip the car seat into the car?
The mothercare Journey will do what you want for 300, though its not very clear whether the seat is compatible with an isofix base or not. The staff in store would be able to tell you. You could upgrade to the Maine seat and isofix base.
Otherwise, I'd look at car seat/base combos first. For example, the Joie Juva and Clickfit base. This base is not isofix, it's belt fitted, but it stays in the car and once installed you probably wouldn't know the difference. Anyway these two will come to around or less than 100. Joie Juva pushchair adapters are the same as maxi cosi, so any pushchair which says it will take a maxi cosi seat is an option and you'd have a budget of 400. If you spend less than 400, you could upgrade to the Joie gemm, which fits in the same base.
If you really don't want to compromise on isofix, you're looking at over 200 for any combination.
Mothercare maine + base 200 - mothercare pushchairs only.
Phil and Ted's alpha + base 200 - looks like maxi cosi adaptors again but I can't be 100% sure.
Joie i-gemm and i-base. Definitely maxi cosi adaptors (baby shop workers often refer to these as multi brand or universal adaptors) - 210
Obviously these prices might be lowered in promotions and sales.
You struggle to find a pushchair with carrycot for under 3-400. Typically the carrycot adds a hundred to the price itself which leaves very little for the pushchair. You might consider looking second hand, with only the car seat part bougtt new. Or, look for a pushchair with the possibility of a seat which parent faces and lies flat, so you can use that as a pram mode from birth.
Pushchairs which take the universal /multi brand car seat clips with lie flat parent facing pram mode:
Maxi Cosi Zelia, 260
Joie chrome dlx, 300
Cybex balios s, 330
Maxi cosi stella 360
Pushchairs with carrycot:
Baby Jogger City mini, + carrycot, 410
Joie chrome dlx + carrycot 400
That's about it I'm afraid, unless you get very lucky with a special deal or multi buy. Keep checking mothercare as they often have special offers where a car seat and pushchair go together for less. January sales might be worth a look.
Good luck! FWIW I looked at the Joie chrome, Cybex balios s and maxi cosi Stella, and loved them.