I have been interested in this too.

The way I did it was just to book the holiday as normal (1 person), but then at the room selection screen make sure you choose a room type that's good for two people (e.g. double as opposed to single).

However, some of the hotels do restrict the rooms so if you are one person travelling, BA Hols may only offer the single room types. You'll just need to shift through the hotel results to find the right one.

I suspect this isn't technically allowed by the hotel as they are only expecting one person staying, but in practice I don't think they will care enough to enforce it. If they do, it might just be to pay for the additional breakfast, tax, etc.