Can I bury the cable to my garden shed 25mts from house -

Yes this can be done, the cable needs to be buried at a depth where it will not be accidentally damaged, also you must mark where the cable runs, so that if some one is digging they find the markers before hitting your cabling. There are other regulations that need to followed when actually supplying the shed. Laying a pipe which will be accessible at a later date will make it easier for a electrician to do the work, and save you time and money

At 12" deep that would be compliant with BS7671:2008. However Steel Wire Armoured cable must be used and the completed job notified to local building control under Part P of the building regulations.

600mm Deep, in sand or in polyamide underground conduit with cable tiles over and yellow warning tape above that. It's deeper than you think and as said above, is notifiable. When you come to the cable there may also be the 'exporting earth' issue to tackle depending on what earthing system you have at your property and if there are any exposed conductive parts that form part of the shed.......External cable runs are often done incorrectly as it is not as cheap as people think to run power externally.