How to spec buried wiring to a garden workshop
Figure out what tools and equipment you'll likely use, and that will largely dictate what size circuits you'll need. For example, table saws come in a variety of powers, but the "hobbyist" ones tend to be on the lower end of that spectrum. In the US, these would be 120V 15A (or maybe even 240V), but that may be different in the UK. Don't forget to factor in lights, AC or heat, general purpose outlets for a computer or music, etc.

Consider which tools and equipment you'll be using at the same time. If it's just you in the shop, you probably won't be using the table saw and bandsaw at the same time. But, you'll probably be using a table saw and dust collector (if you have one). Don't forget about your lights and such here, too. 

When you have those numbers, add up the current or power ratings for everything (using the largest rating for a group of tools which won't be running at the same time - e.g., table saw, bandsaw, lathe). Add some fudge factor to that and round up to the nearest normally-sized breaker (e.g., your tools require 32A, fudge that by 20% to 38A, round up to 40A). Once you have that, you can then calculate the voltage drop and figure out what size wires you need (search here or ask a new question for that).

One big thing to note at this stage is that it's very easy (and relatively cheap) to go bigger. The difference in cost for bigger wires is pretty negligible compared to the labor cost of installing it. So, if in doubt about what size subpanel you'll need, defer to going bigger. I've seen rules of thumb about woodshops doing well with 60A; without a dust collector you could almost halve that.