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.