It sounds like you were trying to book an ex-UK trip with BAH and not use the outbound? As with any flight booking, no-showing any sector results in cancellation of all subsequent sectors, that's just how airlines work and is nothing specific to BAH. You could have tried to book a BAH trip originating from your current destination, so your actual return would be booked as the "outbound" on that trip and you would simply not use the return from the UK. You are unlikely to get any penalty for that. It may not work out cheaper though.

Another point with BAH is that they don't usually sell flight-only, to benefit from their fares you need to package with hotel and/or car. People often do this to get cheaper fares and no-show the hotel/car, which wouldn't usually affect the flight booking.