I haven't tried it myself but tesco do a beer flavored drink called Equator and it's free. Mix it with diet lemonade to make a shandy? X
Assume you'll be taking your own booze?? Malibu and diet lilt is nice. Malibu has 4 syns same as vodka.
Sainsbury's low alcohol Czech lager half a syn a bottle. Well worth it.
Hope the evening goes well
It's 1 syn for the bottle