you can basically have 32a mining in the garage running constantly on 2.5mm ring. but your 6mm 25 metre run can only handle no more than 34a. plus you have a bit of voltage drop. if your antminers leak current over 30ma then your rcd might trip but most probably it wont. you could have another 16mm cable running there supplying two 32a breakers for 2.5mm rings or 4mm radial circuits. though then you need to see your incomer cable size as you might reach overcurrent limit while having a shower on or a kettle etc. if i were you i would just leave it the way it is. just make sure your antminers dont take more than 30-32a.

1400w psu is given with some headroom, though you also have to add inefficiencies.
typical voltage in uk is 240-245v.
1400w/245v=5.71a.
32a/5.71a=5.6

so you can only have 5 of them hashing there im afraid - 5 more if you bring 6mm 32a cable into garage. or 6 more if make it 10mm 40a (you'll need 4mm ring for that). anything more you risk to overload your incomer. though i need picture of it as some big houses sometimes have two or three phase supply coming in but only one is used. also constant current demand might attract the piggies through infrared 'copters as they'll see that your shed is always hot thinking you grow some score.

hope that helps.

being a sparkie and all i might as well give some advice for you mate.  

first of all what type and size of cable you got running into your garage? is it grey twin&earth throught your house or an swa that is routed underground? also tell me how many metres it is? you are obviously dont want to be having a cable getting too hot. take a picture of your incomer head and size of your main fuse too if possible.

Yes. That's basically correct. He's limited by the 100 A service (fuse, not circuit breaker).
His "in house" consumption is limited by the 80A rating of the RCD. However, if he is drawing substantial power in the garage, then it will be limited by the total load on the 100A main switch and fuse.

So, if he exceeds 100A, he will blow out the sealed service fuse, and then have to call the power company to come out, unseal it and replace the fuse and reseal.

The big problem he has in his house is the electric shower. As this has a 50 A dedicated circuit, this suggests that it is a 10.8 kW shower (45A operating load). That is a big load, and once you add lighting loads, kitchen loads, etc. a 40A mining load is starting to look infeasible.

One option is to upgrade the supply from a 230V (23 kW) supply to a 230/400V (70 kW) supply. If he asks his electricity supplier they will quote on the price of an upgrade (usually it's around £2000-£3000), and it'll be another £500-800 for an electrician to replace the fusebox with a 400 V one + another £few hundred for additional circuit installation. In reality, this probably isn't an economically viable option.

Almost. You shouldn't run a circuit above 80% continuous. So keep your 40 amp circuit at 32 amps or less. 32/5.8 = 5x S4s.