An independent car seat specialist will probably check it if you just turn up
But - it’s normal for an isofix base to lift up and down a little - it has nothing securing it down at the end nearest the front seat so this movement is normal. What isn’t normal is if it moves side to side (ie you can pull the base towards you or push it away from you) as this means it’s not tightly enough clicked into the isofix
Either independent specialist, contact ‘the good egg’ car safety, they may be able to recommend a trained fitter near you or there is a facebook group ‘car seat advice uk’ that gives fit check advice based on photographs/ videos you take.
Also, looking up a YouTube fit video may help but obviously not the same as a professional fitter. It’s great you want to get it checked, so many people have car seats fitted incorrectly.