This is a good resource for comparing who seems to offer the best deal:
https://www.uswitch.com/mobiles/compare/sim_only_deals/ you can usually pick up a SIM at most places (newsagents, petrol stations, supermarkets, corner shops, and of course phone shops) and you will need one that you can top up online or with a voucher. There is also some good information in the wiki at the top. Have a good cruise!