It will automatically go Roaming and work on any passing satellite. Mine jumps from Orange, T-mobile, vodafone, you name it and mine uses it, but not cheap!!! Easier to buy a cheap mobile and sim in England to use there.
Netibizas answer is applicable if you are a Spanish person visiting the UK for a short time.
On the other hand, if you a British person and for whatever reason have a phone from Spain, take the SIM card out and put a British SIM card in. Some phones are 'locked' to one company so you can't do this.