Best I can recommend is Auto Finesse Avalanche. Has a nice citrus scent to it, wont strip any wax or sealants, comes up in a nice thick foam and is infused with their Citrus Power which is great for removing grime, salt and traffic film!
£15 for 5L is a quite an ask for a good snow foam, Avalanche is £29.95 for 5L but will last you ages, as the dilution ratio is about 1inch of liquid in a 1 litre snow foam bottle.
It works really well for me and I know it doesnt strip my wax!
Meguiars Hyper Wash is also a good bet, same price also for £29.95. Foams up nicely but doesnt have the cleaning power of Avalanche.
Bertie