UK visa for trailer driver non-EU citizens [closed]

Given that you're not going to live in the UK, and you won't be working for anyone except your Spanish employer, you don't need a work visa -- a Standard Visitor visa will do.

Transporting goods into and out of the UK by road is considered a "business purpose". Or at least transporting goods into the UK is, according to paragraph 21(c) of Appendix 3 to the Visitor Rules:

Individuals employed outside the UK may visit the UK to take part in the following activities in relation to their employment overseas:
(...)
(c) a driver on a genuine international route delivering goods or passengers from abroad to the UK;

Reading this strictly it can appear that a visitor visa is not valid for entering the UK with an empty truck for the purpose of delivering goods from the UK to abroad. On the other hand such an interpretation would seem to fly in the face of common sense -- but bureaucratic regulations sometimes do that, so it might be a good idea to ask explicitly at your interview whether such a run would be a permitted use of the visa.