These are just some recruitment websites for some of the main chains of sandwich and coffee shops in London and the UK. There are many, many more independent cafes and sandwich shops all over the city.
Cafe Nero: https://careers.caffenero.com/
Prêt a Manger: https://www.pret.co.uk/jobs/current_vacancies.htm
Cafe Brera: https://www.cafebrera.com/workwithus.htm
Coffee Republic: https://www.coffeerepublic.co.uk/career
AMT Coffee: https://amtcoffee.co.uk/contact/careers/