I have had mine done more than once at a clinic in Soho - small beauty salon under Smiths hairdressers on Poland St (I think you need to call the hairdressers as they don't have their own phoneline. It sounds dodgy but isn't! Called Smiths Beauty I believe)
They were professional but very cheap (£15? though that was a couple of years ago) I've never found anyone else who wasn't literally four times the price. They do decent facials too, if you want to justify the trip into town.