Your builder/sparky will probably give you some good options and will apply the rules to your property. If you give him your use spec, isolation from kids , load on circuits. Your home set up may dictate the options available/affordable. The workshop consumer unit may be run directly from the main house consumer unit in which case they will have all the above options available, However if its run off a spur from the nearest ring in the house, then your options may be limited or he will ask if you want to go to the expense of laying a new cable.
Sideways suggestion of using 4mm cable would provide future proofing. In my experience as the main user of the workshop, only one tool is used at once so there is little danger of overloading a 2.5mm cable. However one day you might get some 3phase beast of a saw or an electric arc or something needing more beef and 25m of 4mm cable will cost you about £10 more.
As was said earlier a double pole cooker switch would be cheaper than the industrial ones quoted above, but if it were me, I'd spend extra to get the electrics future proofed, especially if you are having it done professionally (as the law requires these days). Your suggestion of an isolation switch seems a good one if others (especially little others) could get access to you workspace.
History: Ring circuits were a British innovation designed to aid post war re-construction, they are resource efficient a they use less copper than equivalent radials, so planet friendly. They are pretty universal in UK and Irish homes and a few British influenced countries eg Singapore and parts of China! They do have advantages such as providing two routes to earth protection, but they are harder to test and if part of it fails they risk being overloaded - suppose you have a 3kw heater on in the winter with something else that is intensive. The sparky will know what is best once you told him your requirements.
I've recently discovered that its the law, or maybe just recommended, that domestic properties have an electrical inspection every 10 years. A few oddities were found in our house/garage, loose connections and a faulty outside appliance, so if you are having work done it may be worth having a full inspection, not cheap about £40 per hour and depending on the complexity of your property would take one or even two days. Oh and the (early 2000s plastic consumer unit now not good practice (route cause of fires) and needs to be replaced by metal - again - maybe we will go back to porcelain...
Good luck Tom