We went with a travel system, so a pram part, the pushchair bit and the car seat. All which can be clipped on and off the base.
We never bothered leaving ds in the car seat and clipping him in the frame, partly because it was the strangest design and you sort of had to tip the car seat forward to get him out :/. We also didn't like to leave him in the car seat for long. With the car seats you can buy a fixed base called 'isofix' base. This basically clips into the base of your back seat and has an arm that touches the footwell. Your seat can then clip into the base. I always found this much safer gab just a belt round the car seat.
We have the pushchair bit, which he was in from about 5 months, and it's great. It can lie flat and he can sit up, so theoretically he could have used it from birth, but he was tiny and I felt he was better in the pram part.
Think our travel system cost about £800 maybe and ds is nearly 2 and we still use it daily. I can't recommend buying a good quality pushchair/pram enough. We walk for miles and ds is always quite happy in his.
If you go into mothercare etc. Have a little push with the prams.Practice taking them up and down, and ask to check they fit in your car (staff will happily do this for you). T
If you don't want to get a travel system you can pick up car seats from birth to about 4 years and those will cost about £200+ and then buy a separate pushchair.
If you use the bus/train a lot, it might be worth looking at ones that are lightweight and can be folded with one hand.
Both mothercare and mammas and papas will do parents events, normally on an evening and they will sometimes have stalls with various venders and you can often get vouchers for pushchairs and other expensive bits.