THere are no on-line HMRC forms for completing trust tax returns. You either buy commercial software or prepare and print a paper one.
Assuming the latter, when you come you come to mail it, you will not be able to buy a post-paid label from Royal Mail because the post code will start with BX and BX codes do not have geographical addresses.
So a trip to the post office.
I love non-joined up government.
Alan