As always, best to have a look at the legislation first, in this case:
The Health Protection (Coronavirus, Restrictions) (England) Regulations 2020:
Restrictions on movement (Section 6)
6.—(1) During the emergency period, no person may leave the place where they are living without reasonable excuse.
(2) For the purposes of paragraph (1), a reasonable excuse includes the need—
(a) to obtain basic necessities, including food and medical supplies for those in the same household (including any pets or animals in the household) or for vulnerable persons and supplies for the essential upkeep, maintenance and functioning of the household, or the household of a vulnerable person, or to obtain money, including from any business listed in Part 3 of Schedule 2;
So you can go out to obtain money.