The problem with analogue is if your equipment has been designed to work in the UK only then it won't be able to tune to all transmitted frequencies in Spain, largely since Spain uses both UHF and VHF, whereas the UK is UHF only, hence the UK purchased equipment may not have a VHF tuner.
Furthermore, whilst Spain uses the same PAL standard as we do, there is a slight difference in the placement of the audio carrier (it's about 0.5MHz away) hence (unless your TV is multi-standard) you may find no you get a picture but no sound, since the TV is expecting to find the audio on a slightly different frequency.
The technical difference is that PAL I is the UK system, whereas PAL B/G is used in Spain - it may describe compatibility in your user handbook.
Newer equipment tends to be designed to work across the whole of Europe and is 'multi-standard' therefore will automatically detect the transmission parameters, or require a manual change in the menu somewhere to work.
Similar problems may exist with the digital receiver, whilst Spain fundamentally uses DVB-T the same as the UK, a digital receiver that has been specifically designed for the UK market only may ignore or have problems tuning to the Spanish digital channels which will carry slightly different service data such as for the EPG etc. Therefore you might find the digital receiver ignores any non-UK transmission, or expects to find certain UK based service data and dumps all the 'foreign' channels somewhere at the end of the EPG in no particular order.
You could buy the tv in the UK and get a Spanish STB to go in the SCART socket on the back.