YY exactly, Toast.
It is possible to get a decent seat which will just do rear facing and forward facing with harness. I don't hugely like the ones which do RF and then FF and then turn into high back booster as well, but two jobs which are fairly similar, ie, a spinning seat up to 105cm from a good brand with a support leg is a fine choice if you want to RF initially and then have the option to change to FF later on, especially if you think you might prefer to FF before the age of 4.
Otherwise, particularly if you are keen on the age 4 milestone for RF, a rear facing seat up to 125cm, which can be more comfortable as they tend to have more child legroom and be more compact in the car than isofix seats, and then swap to a high back booster after this (when you're ready at whatever age from ~4-7) is another good choice, as then each seat only has to do one job each. You can get some excellent HBBs starting at about £100 - don't think you have to keep pouring money into seats. They get cheaper.
There are forward facing seats with harness which convert to high back booster, which is OK as a combination as well, but I would not want to recommend that at 15 months old. It is better to stay rear facing at least initially. Those seats can work for older toddlers (e.g. 3+) who need to vacate their spinning seat for a younger sibling, but still need a 5 point harness, for example.
The Besafe Beyond system is lovely but it is very very expensive especially if you don't already have the base.
105cm is approx age 4 - OP do you know what centiles your LO is on or if they are big/small for their age? If not - what size clothes is she wearing?
Also, do you think you are likely to have another child within the next couple of years? That makes a difference because it makes sense to pass seats down between children rather than try to get one seat which will last your child all the way through and then have to buy a second one for the younger sibling.
Lastly - if your LO is still within the weight and height limits for their current seat then you don't necessarily have to change it just because she is turning 15 months
The ages given are usually a guideline rather than a hard limit (the exception is that babies must not forward face before 15mo).