Yes it will impact your credit rating. Your current country of residence is immaterial to your UK debt. I actually had a similar very similar situation in the US. I left the US to live in Europe and forgot to pay a 90 dollar phone bill and only realized when I ordered my credit report and saw my delinquent bill there.

So yes, if it went to collections it will end up on your credit score.

Another word of advice. In case you ever move back to UK, I would maintain at least one UK credit card and make sure to buy something small (like even 20 dollars) every month in the card and pay it each month.

This will maintain/grow your credit score while you live abroad. CONVERSELY, if you drop off the radar and have no credit-making activity at all in the UK for years, this absence in and of itself will significantly lower your creditworthiness back in the UK.

Again, this happened to me when I spent 14 years outside the US. To my amazement. The credit advisor I hired told me "it is better to have a BAD credit score than a blank credit history."