Technically, yes. If you have a significant or unlimited data allowance with your mobile tariff you could simply take the SIM out your smartphone and insert it into a mobile router.  However, not all mobile operators support unlimited tethering (check your contract), various phone functions are not going to be available on your smartphone while it's missing its SIM, and you might regret the decision if you are overly generous with that shared mobile Wi-Fi connection and run out of data some time before the allowance renews. You can now purchase data SIMs or SIM-only plans with big data allowances on PAYG or contract from all the UK's major mobile operators. Which option you choose is largely going to be based on how much data you need to use, and how frequently you need to use it. If you want something only for when you go on holiday, choose a PAYG plan and pay for data only when you need it. If you want something to get around a flaky home broadband connection or for use on your daily commute, choose either a rolling one-month contract option or save a bit of money with a 12- or 24-month plan.