I would definitely go for a pushchair compatible one. I didn't and I regretted it, as a non-driver - having one which is compatible means that you can get a lift somewhere and walk or get the bus back (or vice versa) and also gives you a method of transporting the car seat if you need to take it with you somewhere. They do have carry handles but they are a bit of a pain to carry for long periods. Of course if you're going on a journey which doesn't require any car travel then you can just use the pushchair or carrycot, which is better for little babies anyway.
Agree with longest crotch-to-shoulder length as possible (or crotch to edge length) - the stage between the baby seats and booster seats is the most annoying for carting around as it's so big and bulky, and harder to fit in as many cars. DS fitted into his seat until he was 18 months, that was a Mothercare travel tot, but I don't think they make them any more, which is a shame as it was a very good quality cheap seat. The other thing to look for is side impact protection, try to find one which has nice deep sides. Some of the really cheap ones have hardly any SI protection, but usually you can get a good deal if you buy a pushchair/car seat bundle. Again it's worth checking out the car seat as well as the pushchair. Agree that there is no infant seat which needs a base - many of them will have a compatible one, but you don't need one.
If you want to think ahead a bit to group 1, you might find that the seat you decide on influences your options with group 0. For example, it might save you money in the long run to go with the Britax First Class or other Group 0/1 seat now (there is one on offer at Mothercare ATM for £40) - but you do lose the flexibility of a more portable seat, they are bulkier to store (and you can't put the baby in it as a rocker) and takes longer to strap in than an infant carrier, and the First Class rear facing position only has one belt routing which is no use for cars with shorter seatbelts. But the upside of this seat is that it will allow rear facing until your child is 13kg, rather than until their head reaches the edge of the seat. DS reached the seat edgeof his infant seat by 18 months, but wasn't 13kg until about 2.5. So an extra year there, and more certainty that your child is likely to get to the recommended 2 years rear facing (recommended worldwide rather than recommended in UK - our shops etc seem slow to catch up to this. My HV told me once DS was 9kg it was unsafe for him to be in a RF car seat!)
The other option I'd say is best for non-drivers in group 1 is to get one of the Kiddy seats or the similar ones, with the impact shield. This is what I have done and I find it a really good seat - it's really light and installs while you are strapping the child in so it takes seconds as opposed to minutes to get it done properly. Safety-wise it's somewhere in between FF harnessed seats and RF seats, which was a bonus for me as extended RF is not really an option if you don't have a car and can't predict which cars you are likely to use in advance.