Britax Evolva is the best one you'll get for under £100, hands down. What of the reviews have you found that are bad? It's decent for safety (not the best, but no 123 seat without isofix will fit that bill) and it makes a good booster seat when you want to move on to that.
If you can stretch to the SICT version, that is better as it has side impact protection built in, which is worth having :) But it takes it over £100. It has isofix connectors, but they are optional.
Another option would be to look at high backed boosters which have the anti-submarine crotch strap thing - Britax (who I trust) reckon that if the child is over 3 and at least 15kg/1 metre tall, then there's no difference in safety between a harnessed seat and a booster seat with one of these straps. There aren't any under £100 but the Britax Kidfix II R is about £115, or Kidfix III M for £130. Again the isofix connectors in these seats (in all high backed booster seats) are optional and fold away into the seat.
It's your choice either way. You'd still have time in an 18kg harness, so if you want to use one then I'd get the Evolva.
Or - wildcard suggestion here - get a Joie Tilt for £69 (Smyths online) and rear face him until he outgrows that by weight or height, then when he has totally outgrown that, get a Britax Adventure, Joie Trillo or Diono Cambria 2 - all of these are £40 or sometimes £30, so fit into your £100 budget, and rear facing is safer than any forward facing seat. You might if you're lucky find the Joie Stages for £100 as well, but at the moment I can only find it for £129. Bear in mind though that this is not a Group 2/3 as a booster, only Group 2, so it will only fit until your child is around 5-6 years, then you'll need to replace it. In this you can use the harness (rear facing or forward facing) right up to 18kg regardless of height, whereas the Britax Evolva is limited to about 105-110cm height, as the harness can only be adjusted up to a certain point and no further.
One last note - you may find seats for this price range with an impact shield. Don't get one. They perform well in testing because they are quite good in terms of keeping the dummy in one place, but in real life crashes they are terrible for internal organs, which crash test dummies don't have.