M&S have never had a full clothing store at Heathrow to my knowledge. Nearest (other than Staines) are Windsor and Uxbridge but all three are pretty small and won’t have the full selection. For that you’d have to head to Central London - Marble Arch, Pantheon (Oxford Street), either of the Westfields, the City. Or out of town to Camberley.

The food stores should accept a Click and Collect order, but best to do a trial run online and see. The garages don’t (or not all at least).