That sounds like a good way of setting the place on fire to me!
If you try to draw around 30A through an unmodified 13A plug the fuse will blow. If you replace the fuse with, say, a metal bolt (which, for the avoidance of any doubt, I am most definitely NOT recommending!), the 30A fuse on the ring circuit will probably hold but as soon as you plugged anything else into that circuit it would then be likely to blow.
Further, if you've used 13A cable to run between the plug and the shower unit it will get VERY hot, as would the cabling feeding the socket itself. A teaching colleague of mine once managed to draw 34A through a 13A socket. The wall in which the cable to that socket ran was so hot that you could burn yourself simply by touching it (so you can guess how hot the actual cable must have been!)
Simply run a 13A extension cable to the shed and plug a kettle in!