Is he just three or nearly 4?
Swivel ones for forward facing are a bit of a waste and a compromise in safety, especially since you're not far off transitioning to the next stage.You can get swivel seats which cover the forward facing with harness stage and the high back booster stage, but they tend to have lower weight limits, have a poor fit for the high back booster stage and if you're paying less than £200 the quality is poor. There are some better rated all-stage swivel seats at about the £350 mark but not for £200, and even these tend to fare less well than a dedicated seat covering one or max 2 stages.
So - personally, I would look for something which incorporates the 5 point harness stage and high back booster stage, with top tether attachment, and make sure you don't miss the top tether when installing (all the following seats I mention have this).
I would normally recommend Britax Evolvafix, however it's over £200 at the moment. Possibly it will go on offer at some point. That has a decent harness capacity (22kg/105cm) and good safety test results. Britax are in general a very good choice.
Silver Cross Balance i-size is also good. The "Essentials" version is £160. The limit is 21kg/105cm on the harness. It's been ADAC tested under another model name and performed well.
Maxi Cosi Onyx or a few of the Titan i-size models are under £200. The weight limit is lower on these at 18kg. But they have good safety testing and are easy to use.
If you want or need higher harness capacity Joie have a new seat called i-Irvana Max which has harness capacity up to 25kg/125cm, so it is a proper extended harness seat with the newer safety standard. Must be the "Max" version to have this feature.
The new ADAC test results are out in the next couple of weeks, if you want to wait for those before deciding. I am not sure whether the i-Irvana Max will be tested as it's literally only just come out so it's probably too late to make it into this round. But it's possible a seat from another brand might be included.