For your first UK return i'd second/third/etc a lot of the advice above about taking professional advice on what exactly you need to do and complete when dealing with UK HMRC.
When I did this many years back I used W T Fry aka "The Fry Group" for the first year but they were not cheap and you ended up doing the paperwork yourself for them to put a stamp on - others may have better recommendations.
There are spreadsheets out there such as "crewtax" that actually take the pain out of the mechanics of doing the calculations, again there are others.