We are not regulated financial advisors in Spain, I cannot legally recommend financial services in Spain or the UK. However I personally used Currency Solutions UK on numerous occasions without cause for complaint. This is not a recommendation just my personal experience, please do your homework, try moneysupermarket and other sites to form a balanced view.
Caveat: Whilst the High Street banks are more expensive, they are more secure, however that security will not extend to the receiving bank or party as a rule. You must check our the laws of the receiving bank.
Tip: Agree a rate for the lump sum, then do it in smaller transfers waiting for each to arrive before sending the next, ie do not put all your money in the system at once. Do your own checks. Give the Banks a chance to match the rates offered by the independent currency traders on larger amounts. Barclays for one will rate match if squeezed on a large transfer.