The main difference between the 5a and the 4a 5G is that the 5a has a larger battery. Same processor, same storage, same RAM, same camera, similar size. Being newer you would expect the 5a to have longer software support if buying now, but that's about it, and if you upgrade after only 2 years then that won't actually matter to you.
I'm not aware of any indication that they will release the 5a 5G in the UK. They stated "US and Japan" when they announced it and cited "global supply chain issues" as the reason not to announce a wider roll-out. The issues with electronic component availability are not going to go away in the next few months, so I wouldn't wait for the 5a here unless you are in no hurry. I would not exclude the possibility that it is never released here.
Of course if you are happy with the 3a why upgrade now? You've presumably got another year of software support, and it's better both financially and environmentally to upgrade less often (switch to a SIM-only contract if you aren't on one already), so what are you looking to gain from the upgrade?
But assuming that you want to upgrade anyway, and you don't want to wait to see what the Pixel 6 brings (it will bring a higher price tag, though as yet nobody knows how much higher), then the 4a 5G seems a good choice. Many Samsung models, including some mid-rangers, are now being offered with equally long software support periods and frequent updates, but you'd have to check for the particular model. If regular updates are important though they are now a candidate (for some but not all models), which is a change from a few years ago. But of course Sammy change the user interface more than most, which is either a good or bad thing according to your tastes (it is however the reason why although they are often very quick with security updates their OS updates come rather later than a Pixel's).