Shimano 5800 full groupset for around £350 from UK is probably the cheapest 11 speed option. Merlin were selling the derailleurs and shifters for £150 to which you'd need to add a cassette and chain minimum, possibly new chainset. I suspect you'd be just as happy with an upgrade to 10 speed Tiagra 4700, which you can pick up for around £250 for the full groupset.