To use my Nexus 5X (smartphone) in the UK, I've ordered a free SIM card from giffgaff (owned by the O2 operator in UK). They say no charge, and free shipping to me in Canada. Unlike many others, one can pay for a month plan with a non-UK credit card as well.

This may be of interest to others travelling from outside of EU/UK. Check out the above link. Pretty decent fees for a month of calls/texts with gigs of data included.