Hmm that's tricky. Graco is mostly Joie, unless you have access to American Graco seats? American spec seats might actually be a good option for you as the harnesses are slightly different plus they tend to have much higher weight limits.
The Besafe clip slides onto the straps, you don't need to unhook them.
5 point plus is worth a try, especially in conjunction with the belt collector maybe, but be warned that it's not totally inescapable - some children just push it down to make the space anyway or they undo the velcro. You might buy yourself a few weeks/months if you never show him that the velcro exists, you can leave it attached to the straps, but he might work it out anyway if he's determined enough.
You can get the impact shield seats in the UK but unfortunately the ones with weight limit over 18kg need the isofix.
In fact, your only EU spec seat with higher harness limit with no isofix requirement is Joie Elevate R129 or Joie Every Stage R129 (Graco Slimfit R129 is the same seat). Joie Elevate is probably going to fit him for longer and is forward facing whereas the Every Stage/Slimfit are rear facing then convert to booster.
Oh, or Maxi Cosi Beryl - this is a decent one with 25kg harness limit and might be able to be brought from the UK but it is a HUGE heavy seat which is a pain.
Look into whether any of the seats on sale near you are US spec. Do your seatbelts lock for child seat install? If not, you'll want a locking clip too. Although these aren't strictly legal to use in the UK, nobody is checking if you are a visitor.
I'd be extremely reluctant to secure a child under 3 only with the seatbelt and particularly one who is so keen to escape! In the UK the seatbelt is actually recommended for age 4+ if possible. If we are talking least-worst scenario, properly secured with booster-adjusted seatbelt is likely better than arms out with harness, but I still don't like it. I don't know where arms-out rear facing sits in this scenario. I know it's less of an issue to be improperly secured rear facing, but of course it's not completely benign either. Carseat.se might have some useful feedback WRT whether arms-out RF is better or worse than such a young child in a seatbelt FF or some useful solutions. I believe they ship worldwide too so try messaging them. They are quite good at the situations where a perfect solution doesn't exist, IME.