As a owner of a Bremont P51 I keep an eye on prices and have seen them go for around the 12k mark. However in the current climate I think your offer was not a bad one, also taking into account a service may be due.
Its a great watch and by far my favourite in my small collection, you won’t be disappointed.

There is a Bremont Town House event on next week in London, it may be a good idea to express your interest as there may be other P51’s available.