yep, law is above. It will be considered a risk.
The value can be anything from £50 to £10000 (and up!) but I suspect a couple of hundred quid. I’m in Somerset, happy to put in my cabinet if I can be of any assistance, however most gun shops will offer a service quite cheaply, and assist with the selling (which you are not allowed to do without a certificate)
As an aside, I am aware of an individual who ‘found’ several hundred rounds of 7.62 ammunition stacked at the back of his garage with some dodgy old 12bore carts. They are all now dropped into the sea (a long way off shore in a firing range) as the disposal would be too difficult as he ‘couldn’t remember’ how he got them.