Costa coffee do a 16floz plastic tumbler travel mug for £3 which is national in all stores. Now compared to a Starbucks equivalent at £12.95 for an identical mug then I think this is a pretty good deal. Having bought the costa mug I can confirm after an 30mins of making a cup of tea with boiling water the temperature remained at 75 degrees, after 1 hour the temperature dropped to 63!
Well worth a purchase. Good availability in all stores