It's quite difficult to find a trustworthy isofix seat for under £100, even forward facing. I do think that forward facing isofix seats are really easy to install, though. It's the rear facing ones which are a bugger as they are massive and bulky, and you tend to have to kneel in the footwell to get to the clips to push them in unless your front seat can totally collapse forwards.
For the "kneel in the seat to get it tight" you can also just push with the heel of your hand. It doesn't have to be a knee, it's just easier for most (women) to get the strength required by putting your bodyweight into it.
I agree with the Britax DuoPlus suggestion as one of the best rated/low cost ones, although it's about £150. Anything by Britax or Maxi Cosi is good. If they have top tether (they will if their car is 2013+), I'd recommend a seat with top tether over support leg as this makes it lighter and less annoying to install/store. Although again these are newer seats and more expensive.
Other seats I'd look at would be:
Britax King 2 (seatbelt install, but no kneeling required) - about £150.
Britax Safefix Plus - isofix using a base system, similar to an infant seat. - about £150.
Maxi Cosi Priori SPS - seatbelt with belt tensioner - about £120.
Maxi Cosi Milofix - isofix and top tether, about £150.
Maxi Cosi Axiss (original, not Axissfix ) - seatbelt install but fairly simple, turns to the side - £160 at Kiddie's Kingdom (otherwise £200)
Britax First Class Plus, or Eclipse might work. Both seatbelt fit but under £100. Require some strength to press into the seat to tighten fit, but fairly simple to fit. I wouldn't recommend the Evolva as the recline isn't good for a 1yo.
Maybe:
Infasafe Event FX? But I don't know much about its safety.
Migo Myla - it's been tested by ADAC and does OK. If you're considering this type of seat, I'm happier to say go for the isofix version of this than the seatbelt fitted one. But I am still not a fan of these seats and I don't really hugely trust them. I'm not sure what the recline is like for a 1yo - possibly not great.
I don't recommend impact shield seats at all.
Second hand car seats can be risky, but if they/you can get a Britax or Maxi Cosi from a trusted friend who you believe about it not being in an accident and it seems to have all the parts with it and the polystyrene is intact, that can be worth doing - ask around as children do outgrow these seats of course. You're looking for someone whose youngest child is 3/4/5 years old.
For forward facing seats it's imperative that the harness is always done up to the correct tension (the manual will explain) and that there is at least 55cm between the backrest of the seat the child restraint is installed on, and the back of the seat in front. If you can move it forward far enough that the child can't kick the seat, this is usually far enough.