A long-lasting price but still an extremely good deal. Available at all company-owned stores (most older ones on high streets/shopping malls etc). Around 90% of total UK stores do this. Franchise stores (e.g. owed by Euro Garages, many drive thrus are like this) sometimes do and sometimes don't. The coffee is £1 but you get 25p off with your own tumbler/mug. You also earn a star if you pay with Starbucks card. You can also have hot milk made to put in it or pouring cream free. Also cinnamon, chocolate, vanilla and nutmeg powders all available to help yourself to at the sugar servery.
Here is confirmation of the free refill if you sit in with first drink. https://customerservice.starbucks.co.uk/app/answers/detail/a_id/3854/kw/filter%20refill
Often near the end of the day they turn the filter machines off and will make you an (IMO superior) Americano for the same price.