Loads of options.
No one provider guarantees coverage and there will be places where it is limited to non existent.
All the main companies have their own stores, Three, o2, Vodafone and EE plus carphonewarehouse sell a range of phones and SIMs. Phones and SIMs can also be bought in larger supermarkets and other retail outlets.
If your phone is unlocked then you could just go with the SIM only option.
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