My understanding (from the RYA website and materials I have) is that the Yachtmaster Offshore can be obtained by having the following experience "50 days, 2,500 miles including at least 5 passages over 60 miles measured along the rhumb line from the port of departure to the destination, acting as skipper for at least two of these passages and including two which have involved overnight passages. 5 days experience as skipper. At least half this mileage and passages must be in tidal waters. All qualifying seatime must be within 10 years prior to the exam."
. You also (obviously) have to pass their theory & practical examination.
Whilst I am told going through Day Skipper course is good, I haven't once been told it's a requirement (although they keep strongly implying I should take it anyway). As such, I strongly believe that it's just the experience you need.
You will need a VHF radio
license and a first aid certificate though.