We'd advise looking for a deal from a well-known mobile operator that has reliable coverage in your area (or your holiday destination), with a big data allowance, no restrictions on tethering, free or cheap roaming, and a low monthly or PAYG cost. You can check network coverage in your area for 2G, 3G, 4G and 5G on the mobile operator's website. Use the following links to jump straight to a mobile operator's coverage checker: Vodafone, EE, O2, Three, GiffGaff, Smarty, VOXI, iD Mobile, Virgin Mobile, TalkMobile.  There are many more companies offering data SIMs than we have listed here, but all are what are known as MVNOs, or mobile virtual network operators. This means they piggyback on another mobile network (Vodafone, EE, O2 or Three) for connectivity. As long as you have coverage with the big four, all the MVNOs should be covered in your area. The best data SIM deals are always changing. We've collated more in-depth advice in our frequently updated guide to the best SIM-only deals, but our preference currently falls to Three. All its SIMs support 5G. At Three you'll find a range of SIM options including unlimited data from £22 per month, or you can choose one of four PAYG packages that are valid for up to 12 months. For the latter, 24GB of data costs £60; 12GB of data costs £40; 3GB costs £16; and 1GB costs £10. Smarty, which also runs on Three's network, currently has a great deal on 50GB of data for £10 on a12-month plan. Tethering is unlimited in the UK.