Is she in an infant carrier seat at the moment? That's probably the best thing to keep her in for now as it will give you more options if you wait until closer to the end of the first year to change.
Do you want to move to a rear facing seat (safer) or forward facing?
Do you have any specific issues with your car e.g. little space, or very sloped back seats, or storage boxes meaning you can't use a seat with support leg?
Is your daughter big, small for age or around average?
You say you want Isofix, but that is a little limiting in terms of options particularly if you want the safest seat you can get. Bear in mind that for the next stage you tend to fit the seat once and then leave it in the car all the time, so the method of fitting isn't quite so crucial.
Having said all that with no other information to go on other than an Isofix seat for a 6 month old for £350, I'd suggest Britax Dualfix i-size M.
If she is bigger for her age, you can compromise on the Isofix point and are happy rear facing, Avionaut Sky.
If you don't want to do extended rear facing but she is bigger for her age, Maxi Cosi Beryl, but I'd recommend you max out the rear facing limit on it (up to 13kg).