She still needs a 5 point harness as she is under 15kg and 100cm - these are the two minimum stats for using a high back booster with adult seatbelt. It is better arguably to wait until the child is closer to 18kg, but given she might not be there until she is more like 6 or 7, I think it's a situation where you're OK to wait for the minimums, but she does need to meet both of them.
Do you know if you want rear facing or forward facing?
For rear facing I'd look at Britax Safe-Way M or Axkid Minikid 2 as these are on offer currently and are good value. However you can't get them from Halfords - you'd have to argue to swap the voucher, or spend it on something else e.g. a bike, roof rack etc and divert the funds for the car seat.
If she stays on the same centile lines, they will last her until she is about ten (which, obviously you don't NEED to rear face at age 10 so they will essentially last as long as you like). They are seatbelt fitted, but combined with the Swedish type tethers and the support leg they are extremely secure.
OTOH if you prefer forward facing or want a seat which will go all the way, these are the only three I really like:
Britax Evolvafix/Advansafix
Maxi Cosi Titan i-size
Silver Cross Balance
Halfords have the Maxi Cosi Titan Plus i-size, Titan S i-size, and Onyx i-size which looks similar but is new to me. I would assume it is up to Maxi Cosi's usual high standards, and the harness looks tall like the Titan seats. They don't stock the Britax or Silver Cross seats at the moment (sometimes they seem to do the Britax one).
These seats last up to 18kg/105cm in the harness, which should probably get her to about age 5.5 by which point she should be heavy enough to go into the high back booster mode. They then go all the way up to 150cm (or likely she will reach age 12 first).
Isofix is definitely recommended for harnessed seats forward facing, especially with a top tether or support leg. These seats all use top tether which means that the crash performance is better than a seatbelt fitted seat. If your seat was moving around when fitted, then it's possible it may also have been too loose - isofix eliminates that possibility because it's a solid connection to the car. So definitely recommended 
It is a bit more expensive than the Stages (£179 for Onyx) - if you need a cheaper one, your options start at £60 for Joie Elevate R129 - this seat is very basic, but it's the best one of the cheaper models because it has a well-positioned belt guide for the high back booster mode. However, it is seatbelt fitted in 5point harness mode, and you might have a similar issue with it moving around if the seatbelt buckles in your car are further forward.
Graco Slimfit R129 is worth a look for £100 if you want to look at rear facing - this is RF up to 105cm/21kg and then forward facing once the child is over 100cm and 15kg (but only in high back booster/seatbelt mode). It got a reasonable safety rating on the ADAC test, but it's not isofix. Similar to the Joie Stages if it didn't have a forward facing harness mode.
Graco Energi R129 (£120) is probably the cheapest isofix type seat I'd look at out of the options here, again because the belt guides are well-positioned and it's lightweight which is helpful for the high back booster mode. Or Joie i-Irvana is similar. Honestly though I do prefer the Maxi Cosi to these models. I would only look at these if the Maxi Cosi would be a stretch financially.
They do have a 10% off code at the moment so it's possible the Onyx could be closer to £162.