I'm not a fan of the Joie Elevate and although the r129 version is probably better, it's still not what I'd opt for ideally I don't think. It's really prone to forward anchorage (it even has it in their own fitting video) and that makes it loose in a lot of cars.
If you do get that one, get it somewhere you can test the fit first and check that it has no movement (pull on the harness when installed). And I agree that the R129 version is a better buy for £80 than the original. Looks like John Lewis, Smyths and Halfords all stock it potentially.
That one's £80. So for a similar price, up to £100 you could compare to:
Kinderkraft Safety Fix or Safety 1st Ever fix - quite basic seats/basic brands, but both have the isofix and top tether, which the Joie one doesn't. 123 seats (convert to booster) OTOH, these are both the older standard, whereas the new Elevate is R129 (better safety testing). If I had to guess, I'd say the new Joie will be better for side impact protection/better as a booster seat, whereas the tether seats will give you more safety in the harnessed mode. Top tether 123 seats tend to get reasonable safety scores, even from the cheaper/less safety conscious brands (like nania).
If you can go up to £150 you could look at:
Maxi Cosi Axiss which lets you keep the rotate function even though it's only forward facing. Belt fitted, with tensioners (means you are likely to get it tight enough easily). However, only up to 18kg. OK safety rating. It has a recline (the elevate doesn't)
Maxi Cosi Tobifix. Isofix, forward facing, only up to 18kg, but good safety and recline function.
Britax Evolva SL SICT which is a 123 seat (turns into booster) with good safety rating. Likely substantially better than Joie Elevate, probably better than the Kinderkraft or Safety 1st. Seatbelt fitted.
In the event you can go up to £200 or wait for sales, you could keep an eye out or look for older/more basic versions (I found options for all of these currently on sale less than £200, if you want links to any): Silver Cross Balance, Britax Advansafix, Maxi Cosi Titan or Recaro Tian - all of these have very good safety compared to other 123 type seats, all convert to booster so will last you a while. Sometimes they go even cheaper (up to £150ish) especially the older Maxi Cosi Titan models. Maxi Cosi in particular often do the Black Friday sales in November, and there are often end of summer sales around the Aug Bank Holiday (28th this year).