The ITV is the MoT equivalent. you take it to a test station (not a garage, purely do tests). First one is due when car is 4 years old, then every 2 years until it is 10, then every six months.It seems that the clock starts ticking from first registration, even if the car is not new.
ITVD is the road tax, due from whoever owns the car on the 1st January each year but usually to be paid later in the year - usually best to arrange a direct debit with the town hall or DGT website if you have a digital certificate.
In my opinion it is best to sell the car in the UK and buy one here - you would be sat on the left, so safer, and although prices seem higher than in the UK the depreciation is way less. You don't not buy a pint in a UK pub because it is cheaper in Spain, so just accept car prices here as one of those things.