The Cybex Sirona is still a good model, but if 2yo is on the larger side it might not last long enough with 105cm limit. However it does have a 20kg weight limit which is good - some of the 105cm limit ones are up to 18kg only.
The only seat Halfords stock which has a larger RF limit (125cm/22kg) is Joie i-Prodigi. This is an online exclusive so you won't be able to try it in your car, but if you have had a Cybex Sirona in it, the Joie should be fine. However, it is £430 which is significantly more expensive than Cybex Sirona Gi, so the voucher might not cover the full value.
Halfords do stock the Sirona Gi - if you opt to go for the same one again, be careful to avoid the Sirona G, which is not the same seat, it's the modular version which goes onto a separate base and cannot be fitted without the base. So be careful and don't buy this accidentally, as you'll have to spend a further £130 on the base to fit it. The Sirona T Plus is the same story, but the base is £200 
If you fancy having a bit of an argument you could possibly insist that you prefer to buy from wherever you originally got the Cybex seat from, due to wanting to have it fitted into the car by staff you trust etc (just ham this up a bit even if you don't think it's true) - the insurers often prefer to give Halford's vouchers, but if you can push for cash value instead, it means you can spend at a different retailer, perhaps one who stocks higher limit RF seats for less than £400. There are a couple around £200-300, and they have higher weight limits than the Joie seat as well.
If you can't top up the voucher and need a cheaper seat from Halfords but don't want to go for a 105cm limit one, the best one for a higher centile child is probably Joie Bold 123 (not i-Bold) because it will keep them harnessed up to 25kg, although not an ideal solution because it is a forward facing only seat.