If Tesco does not support the Apple Watch, then they have not contracted with O2 for the privilege of sharing the iPhone number with the Apple Watch.
Basically, YOU CANNOT enable the cellular radio in a cellular Apple Watch as long as your iPhone uses Tesco.
You can switch your iPhone to a carrier that does support the Apple Watch
Apple Watch - Carriers - Apple
Or you can get a GPS Only Apple Watch, which as long as your iPhone is nearby, will do everything a cellular model will do, except it will take advantage of the iPhone cellular radio. Where nearby is both are somewhere in the same moderate sized wood frame constructed home. A GPS Only Apple Watch is less expensive, and does not have any monthly charges.
If you get a cellular Apple Watch, you can use it as a GPS only model, and when you do switch your iPhone to a carrier that supports the iPhone, or Tesco gets a contract with O2 that allows Tesco to support the Apple Watch, then you can enable the Apple Watch cellular radio. You will pay more for the cellular model, and when you are able to activate the cellular radio you will pay an additional fee to the iPhone carrier for the service (it would be $10 U.S.; not sure the UK amount, but in that range).