I have shipped to the UK a few times, never had any customs issues. The only problems I have had are the delivery times: Royal-Mail is very slow lately with international shipments, but nothing else.
As
@eBoy said, fill out the customs declaration properly. Write "watch returned for repair" or "defective watch for parts", value 30-40 €/$/£.
The risk is always there, but it's not that high.