I have been asked this quite a few times recently. so I though that I would give a shout out to the only two have ever really used.
The first is Compton Cycles over in southeast London. Owner, Jim is a true gentleman and in addition to using them for repairs, a Brompton service and actually buying a Brompton from them (way back in 2013) if I were to buy a Brompton from a bricks and mortar shop it would Compton Cycles. Actually there is one more.
Over in Chobham, in Surrey is Brilliant Bikes. They started out as an online parts business - and to this day is THE best location to buy anything Brompton spares. They are just...brilliant! It is likely you might encounter Mike, who is very much in the same mould as Jim over at Compton Cycles.
Brilliant Bikes also have a YouTube presence and a search for Brilliant Bikes will reveal lots of reviews, helpful tip/tricks and many highly entertaining.
Just in case you are wondering, I have not been paid or get a discount from Compton Cycles or Brilliant Bikes (mores the pity) and I write this as a happy customer. (In fact I know lots and lots of equally happy customers from the Brompton fraternity).
Stay safe out there people!