The ticket terms and conditions state it is prohibited to bring "alcoholic beverages and foodstuffs (except those permitted at the discretion of PGA European Tour);"
In last year's 2023 British Masters FAQ they said: "No picnics / picnic hampers allowed. Alcoholic beverages are also not permitted to be brought into the venue."
Probably some snacks won't be an issue but they may object if they do a bag check and feel you're taking in too much. You could email their ticketing team to clarify: ticketing@europeantourgroup.com
They have a water station and encourage to bring reusable bottles.