If the rent, or at least part of it, is based on turnover (which is often the case with retail leases) then the lessee will in any event pay less rent. However, in respect of any base rent (not connected to turnover), there is unlikely to be a right for the lessee to seek a reduction under the terms of its lease. There is nothing in UK law (outside of the terms of the lease) that would give the lessee this right.
In the event that a lessee fails to pay rent there is now a moratorium on the forfeiture of commercial leases. This means that lessors cannot currently forfeit commercial leases until at least after 30 June 2020. This date may be extended.
In addition, on 23 April 2020, the Government announced a suspension of the ability for landlords to use statutory demands and winding up orders if a tenant cannot pay its rent due to Covid-19. This measure is initially intended to be in force until 30 June 2020, but can be extended in line with the moratorium on commercial lease forfeiture. Legislation has also been announced to prevent landlords using commercial rent arrears recovery (CRAR) unless 90 days or more of unpaid rent is owed.