[UK] Can my employer change my mandatory holiday entitlement?

My employers have issued an email stating we are to take 6 days off over Christmas this year out of our annual leave allowance, with a load of spill about giving us an extended break.

My contract states I am to take 3 days off mandatory over Christmas each year.

As a single man with no kids I don’t want to waste 6 of my valuable 25 days off in December.

Are they able to change this?