But then if it's triple the price for a much faster, potentially anything from 4x to 10x depending on what ADSL speeds you're getting, then I'd say that's worth it. With the number of online orders, reservations etc. a central London Restaurant will be getting I'd say it's worth it. I struggle to comprehend how a business with an online element of any sort could survive on ADSL these days, especially with the dreadful Upload. And surely a business ADSL service is still in the realms of £40-£60pm? Add in an additional service for Bonding (but still only Upload limited to around 1.2-2.4Mbps if the bonding works well) then you're looking at £80-£120pm. In Central London i reckon you could get a dedicated Leased Line, 100Mbps both ways with minimal to no install costs for just over £100pm. (+VAT but the business will be claiming that back.)
Although we don't deal with them at Sky (and I can put you in touch with multiple providers that we do deal with if required and if there are multiple sites a deal may be possible on pricing too), the only one I know of that advertises prices on their site is Spitfire.
https://www.spitfire.co.uk/connectivity/fttp-ethernet/ who are offering 50Mbps symmetrical for £100pm.