Base for decking

Decking supports are typically mounted to concrete piers... you can make your own but it's far easier to buy them ready made. I prefer the floating construction rather than having a deck bolted to a house. Pea gravel under a deck will retain moisure and present a mosquito problem... gravel should be illegal as a landscape material anyway, it migrates everywhere.

Where are you?

If your ground freezes you need to dig holes below the frost line and pour cement into sonotubes.

By using floating pier construction that's no longer true.

In most jurisdictions, building a deck requires a building permit: and the local permits office can usually advise free about alternative foundation methods (e.g. deck blocks or sonotubes. The former are preferred here on sand and Leda clay, despite frost a yard deep in winter.)

He's obviously in the UK.

The OP is in the UK. There are few places in the UK that receive hard frosts let alone heaving freezes. And were there zoning laws there about how one must fabricate deck post anchoring he'd not be asking here.... he's merely asking for DIYer recomendastions based on folk's direct experience. In the US a permit for adding a deck is for property tax purposes unless it involves swimming pools/hot tubs, etc., typically no one from the building department comes out to inspect deck fabrication unless it's physically connected to the house (one of the main reasons floating piers were developed), only occasionally do they manually measure square footage, but usually additions/area is determined from periodic aerial photography. I doubt many if any municipalites in the US have general deck code per se, only in special circumstances such as set backs from property lines, and as most DIYers don't apply for permits the zoning officer will catch you later, usually a couple three years later and then the town will reassess your property and make it a bit to the high side so as to collect for the years you cheated. They'll rarely fine the DIYer but they certainly will a contractor.