I went on holiday, came back and my Samsung S8 no longer connects to any network.

[JOANNNA]

I was in Japan for 10 days on flight mode and when I touched down back in the UK and turned off flight mode, my mobile data and signal didn't return. My sim card works in other phones. I've turned it on and off multiple times, taken the sim card in and out, ordered a new sim card, did a network reset and even a factory reset on the phone but no luck. The rest of the phone is working completely normally. I don't understand how this can be possible? Any ideas? 

[BandOfBrothers]

It's rare but I've seen this happen before @JOANNNA  where a person has gone abroad and then the phone has glitches when back in the hone country.

Can you manually check the Apn settings in your phone to ensure they are 100% correct for your UK network  ?

You've performed some good troubleshooting there so now if this was my phone I'd visit a Samsung Experience Store or Samsung Service Centre for assistance.