Hello, The Devices may be physically different but they may have the same CPU, if they are the same, you need to try to find from both OEM firmwares (UK A5 V4 and EU A5 V4) if they are the same, after checking, comparing and confirmed that you may be able to try to install at your own risk the prebuilt image for the Archer C50 V4 Via Tftp. for your Archer A5 V4 UK Version.
Mine was the US Version Sold by Amazon and worked very Well. Just the 2.4 Ghz Is still not fixed and that wasn't a problem for me since i use only 5Ghz and the Ethernet Ports.
The firmware may be old since the one who made the prebuiilt image did like 1 or 2 years before (For the C50 V4), and the last one, he made just one recently ( as i requested him via PM )so they may differ, and of course you can't use the prebuilt image for my A5 V5 for your A5 V4, So you may need to install the latest sysupgrade after sucessfully installing the C50 V4 Prebuilt image.
Any Doubts, please Let me Know.