In addition what Err_404 wrote, you should keep the warranty situation in mind:
Nothing’s warranty is only valid in the country of purchase, so if you get a new NP3 or even 4a during your UK trip and there is going to be some sort of issue down the line which could be covered by warranty, it would have to be claimed with Nothing UK.
Also keep in mind the 4a will be launched on March 5th, not released? Usually they will launch and release about two weeks later, so 4a would be available by mid March.
Regarding trade-ins, Nothing doesn’t do these directly, at least here in Germany sone tech retailers like Media Markt are offering their own trade-in services however, I don’t know if something similar might be available in the UK.