If you are in furlough you are still considered as working full time for your employer and therefore still accruing holiday pay entitlement.
If you decide to take or you had already booked holiday, then you should take it. Just because you are in furlough it does not mean you may not wish to take your holiday.
When taking your ‘holiday’ you are entitled to 100% of your holiday pay not 80% furlough pay.
You employer will still be able to claim 80% furlough pay back but they must fund at least 20% to take you up to 100% pay.
Amazingly, if you are in furlough for a long period, when you do return to work you will still have your full holiday period entitlement remaining or the balance of this if you had taken holiday before lockdown.