You can buy a Brompton and spare parts from quite a few bike shops now. Brompton have their very good 'Brompton Junction' shops dotted around the world and you can now even buy direct from Brompton themselves. There are of course a few alternatives.
Over at Catford Hill in southeast London you have Compton Cycles. Owner Jim is a king among men and it is a shop that you really will be looked after in a way few other shops can. Compton Cycles was the shop I bought my Titanium Orange Brompton and my Surly Disc Trucker. If I ever bought a Brompton from a bricks and mortar shop it would be joint top of my list.
Over in Chobham, Surrey sits Brilliant Bikes. They started out as a mail order company where you could buy pretty much every Brompton spare part you can name - in fact they still do. They now have a physical shop. I have taken my Brompton bikes there to be serviced and like Compton Cycles, they really do look after you! Mike can usually be found there and again like Jim at Compton Cycles, he is brilliant. In fact everything about Brilliant Bikes is brilliant.
If you search Brilliant Bikes on YouTube you will find lots and lots of helpful, entertaining videos about all things Brompton. They really are worth a watch and the lady who presents them does a wonderful job. (Some have out takes which are equally as entertaining).
Both shops do of course sell other bikes apart from Brompton but the small-wheeled wonder is what I know them best for.
Now, in case you wonder, I have not been paid, threatened or cajoled into writing this. I recently placed an order with Brilliant Bikes which as always got to me in record time. When I thought about a blog post about the best bikes shops that do Brompton, I had to mention Compton Cycles too.
Stay safe out there people!!