When sizing a submain feed for a workshop you need to know a figure for maximum demand. This is the most 'electricity' that you will be using at any one time. Take your most powerful tool and add on the extractor, lights, radio, chargers and any other electrical appliance that you might have on. In my workshop the worst case scenario is when I have my mitre saw (2000w), trend extractor (1200w) and my dust extractor (1500w) running at once.
As voltage drop occurs throughout the entire installation it's a good idea to allow yourself some spare capacity in the cable run you are calculating. IMO, a drop of 4V is the absolute maximum for a submain feed that is likely to supply loads with startup surges. Further VD will occur in the cable between the saw and the consumer unit, and between the service head and your house consumer unit, and the total drop should not exceed 9.2V.
Take a maximum demand of 20A over a 30m run. The figure for VD on 6mm² SWA=
7.3mv/a/m
7.3mv * 20A * 30m = 4.38V
10mm² SWA=
4.4mv/a/m
4.4mv x 20A x 30m = 2.64V
As I said earlier, I would consider running 10mm², but if the cost is prohibitive (30m 3core SWA = about £110) then 6mm² should be ok.
Oh, and as OLD pointed out, there is a requirement for RCD protection for circuits likely to supply outdoor appliances. For a domestic outbuilding it is reasonable to expect that this is the case so if you do not have 30mA RCD protection either in the house that protects the submain, or an RCD in the workshop CU, then this should be seen to at the same time.
