A 50 seater at Heathrow is extremely difficult to operate profitably on a consistent basis. Flybe were forced out of Gatwick for operating 78 seaters.
The niche I can think of where this might work on a scheduled non-charter basis is a business-class-only B737 flying 5+ hours. There are perhaps other profitable niches for an aircraft with just 50 seats, but they are very much highly specialised niches.