You could explore your way around the various ammo manufacturers web sites and see what they offer, I've just looked at Federal as an example and they don't make anything lighter than 150 grain that I could see. There is a 123 grain Sako load that is very popular indeed with many highland stalkers and it is the lightest I know of and I can also be confident that it works well on deer.
Your problem in the UK will be that you might find a load that takes your fancy but getting someone to sell it to you, especially getting a reliable and regular supply, will probably be difficult. You would also need to be careful that lighter bullets were suitable for deer, if that is your intended target, and not just frangible bullets designed to destroy small vermin.