I lost my sim while travelling, can I get a new one in a shop? I land at Heathrow next week

[JJP2RidesAgain]

https://help.smarty.co.uk/en/articles/1215834-what-to-do-if-your-sim-card-gets-lost-or-stolen 

I would order a replacement SIM card ASAP, which will be posted to you, and should be waiting for you when you arrive home :

EDIT : The link appeared above my comment. 

[MSF]

@PhilipM_1923321  You won't be able to use one that you buy in a shop. As @JJP2RidesAgain  says, you need to report the sim as lost/stolen. The replacement that comes out will need to be activated but will still have your number and Smarty plan attached. Your account will be suspended for your security until you activate the replacement.