In simple terms, based on my understanding, in theory HMRC should be made aware of Joe Soap spending over a certain amount within a 12 month period. I'm pretty sure that amount is £10,000.
I sold a property in the recent past and sank a figure a good bit North of £10,000 into gold, in multiple purchases in the single figure thousands with the same dealer. A dealer who I have a long standing relationship with. Not heard a squeak or received a strange probe from Hector the Tax Inspector.
So long as you're not knocking out dope and laundering your ill gotten gains there is nothing to worry about.
Any business man these days tends to be wary of a man trousering a large envelope of cash. The days of rocking up at car auctions, spending 20, 30, 40 thousand and a large brown envelope being accepted are long gone! Use bank transfer I would suggest then you will have a nice clean traceable record if the tax man takes interest and money grabbing bankers won't steal their "share".
And welcome to the soon to follow addiction of shiny stuff!