Depends - if it's a minicab (private hire) company and they have quoted a fare then the cost may be exactly the same, or they may add an extra person charge of (perhaps) £1.
If it's a black cab (licensed taxi) then they will charge using a meter (so the £10 may only have been a guess anyway) and this will include a surcharge for each person travelling - the precise amounts differ depending on the licensing area.
Either way it won't be twice the single person fare - more like an extra £1 or so I think.