The head flop is most likely to do with the slope in your car - you will struggle to avoid this with an isofix seat, except for Besafe izi Turn (or Twist).
That one is on a Black Friday offer for £309 in some colours, so might be worth a look. It can accommodate any slope in any car, so can buy online.
However, be aware that if she stays on her current centile lines, she is likely to outgrow that kind of seat (including the RF mode/forward facing with 5-point harness mode on the All in All) before she turns 3, which is too young to move to a high back booster with just the adult seatbelt, so you will end up needing to buy a longer lasting extended rear facing seat or extended harness seat later on. Not a huge issue if you intend to hand down to a younger sibling, but annoying if you don't. It is possible she will slow down in growth a bit esp if she hasn't started walking much yet.
Other good spin seats on offer, will have the same issue of the height/weight limit being limited - would try to test these in your car before purchase, to see if they have the same problem re angle, so I have only listed brick and mortar store offers.
Cybex Sirona Gi i-size - £195 Halfords (slightly higher weight limit, so might last a few months longer)
Joie i-Spin 360 - £178 John Lewis
Joie Spin 360 - £130 Halfords
Joie i-Pivot - £149 John Lewis/Mamas & Papas
Graco Turn2Me i-size - £130 Halfords
Maxi Cosi Mica Eco - £149 John Lewis
In order to avoid the early outgrowing issue and also the isofix issue you could also look at these extended rear facing seats which last much longer and where the recline works differently so will be less affected by the slope of your seats - would recommend looking at local retailers if you can or looking at a video to see how they install, as it's totally different to isofix. Any of these would last until at least age 5-6, possibly longer (much better age to move to a high back booster). They are also MUCH nicer to use than the Cosatto seats, which aren't designed well with user friendliness in mind at all.
Klippan Century or Opti 129 - £125/150 at In Car Safety Centre
Axkid Move/Minikid 2 - £180/£225 at Axkid retailers
Then if you did want a longer-lasting isofix one again with a good recline you can still find the Axkid One2 in some places for £499 and even in Cosatto colours for £525 - I appreciate this is a lot more than the other seats I've listed, though! It is a very cool seat with really amazing features, but a lot of money as well.