I wouldn't get this seat. If you look at the ADAC crash testing results it doesn't come out very well - only Average for both front and side impacts. Those results have only just been published so you might not have seen them before.
If you want a forward facing 123 isofix seat the Britax Advansafix IV R is much better performing, and it's the same price. I'd definitely consider this seat e.g. in the situation of a ~2.5-3yo, average size or smaller child needing to vacate a more protective seat for a younger sibling. However in most cases my advice would be don't go for a forward facing Group 123 seat at all.
Firstly: I would skip the up to 12 years requirement. Reason being seats which encompass both a booster seat and other modes just make too many compromises and don't perform that well in either mode, and plus a booster seat isn't that expensive to buy anyway. I'd take £40-100 out of your budget and put aside for a high back booster in ~3 years' time (that will easily get you a good quality one) and look to spend £200-260 on this seat for now. (More reasons: If you have a second child, you can probably use the more expensive Group 1 seat for them in turn rather than having to buy another, in 3 or so years' time there will be even better booster seats on the market, and the total lifespan of your booster seat will be increased by not first having been used as a harness seat for 3+ years.)
Secondly: If this is for a child just coming out of the infant seat it's much much safer to look at seats which can keep them rear facing for longer. The new recommendation is to keep children rear facing until at least 15 months old (it's a bit confusing, but in some seats this is actually law whereas with others it's still a recommendation). But I'd actually try to exceed that and go a fair way past a year. If you want to be really safe it's actually possible to get seats which can rear face right until they are out of a harnessed seat entirely. And many on the market which have the option to do either direction, which means you can make the choice when you feel ready.
Your other requirement for isofix is absolutely doable. The only thing I'd want you to know about in advance of making the choice is that the stage after the baby seat comes in two sizes - Group 1 (up to 18kg) or Group 1/2 (up to 25kg). A Group 1 seat will fit a child roughly until they outgrow age 3-4 in clothing, so if your child is large for their age, you may get to the end of Group 1 and go aaargh what do we do now, we need a bigger seat and end up needing to buy a Group 1/2, which will last roughly until the child outgrows age 6-7 clothing. That's expensive and so you might as well just get a Group 1/2 in the first place. Also, if you like the idea/option of your child staying in a harness and/or rear facing for as long as possible, then you might also want to go straight for a Group 1/2 seat rather than outgrowing the Group 1 first.
Seats to look at in budget:
Group 1 isofix spin seats (all have a handy spin feature to get child in/out, can be forward or rear facing up to 18kg - my top vote)
Cybex Sirona Z
Cybex Sirona S
Britax Dualfix
Nuna Rebl Plus (John Lewis sale)
Joie 360 Spin
Group 1 non spin isofix seats:
Concord Reverso Plus (Rear facing only)
Maxi Cosi Milofix (RF to 13kg, FF to 18kg)
Hauck Varioguard Plus (only the Plus one) - RF or FF to 18kg
Forward facing Group 1 seats with isofix performing better than the Titan:
Britax Trifix 2
Recaro Optiafix
If you think you might need/want a Group 1/2, you won't get a pure isofix one because isofix has a weight limit which is too small for this type of seat. So if you are worried about fitting, you will want to go to a specialist retailer: www.carseatadvice-uk.com/specialist-retailers.html
Also, apart from one, they are all either forward facing only or rear facing only. So you'd have to choose.
Other than that the seats in budget would be:
Rear facing only:
Axkid Move
Britax Max-Way Plus
Britax Multi-Tech
Rear and forward facing:
Diono Radian 5
Maxi Cosi Beryl (Out of budget, only RF to 13kg - but worth a mention)
Forward facing only - actually these both convert to boosters as well. But the same cautions apply.
Joie Bold
Cosy n Safe Excalibur