Being sponsored to do an activity for an hour a week over the period of the volunteering section counts towards the award as you will be supporting the BHF by raising funds . One way to cover the duration of the volunteering section is to do a sponsored activity for the 12 weeks e.g. walk (or run, cycle, swim etc.) for an hour a week and raise sponsorship from family and friends.

To find out more you may like to look at this Just Giving page you will see some examples of how DofE volunteers are fundraising for their DofE. You can upload progress reports, photos and you can share the link with your assessor so they can follow your activity too.
https://www.justgiving.com/campaign/dofe-fundraising-for-bhf

There are lots of other fun and unique ways to fundraise for the British Heart Foundation, and you could mix a few fundraising activities – this page is a good place to start
https://www.bhf.org.uk/how-you-can-help/fundraise/do-your-own-thing