Can I leave Frankfurt airport during a 10hr layover between Namibia and the UK (during covid!)

First things first: you obviously need to be permitted to enter Germany. If you hold UK citizenship, that is not an issue; if you hold Namibian citizenship you need a visa, for other citizenships you would need to check.

The Covid prevention rules are volatile and could change at any time, so it is imperative that you check they still apply when you are actually travelling. As things currently stand:

• the UK is classified as a high-risk area;
• Namibia is not classified as a risk area.

This can be checked on the RKI's list in German and in English.

In addition to the designated risk areas, entry rules are also volatile and may change on short notice. Currently, the situation is as follows:

• Unvaccinated travellers must present a negative test result (antigen tests suffice). This may also apply to those whose second jab was more than three months ago and who do not have a booster jab or have a cured certificate.
• Travellers from high-risk areas or virus variant areas (who visited these areas in the preceeding 10 days) must fill in a digital entry registration form prior to entry. Unvaccinated travellers must also remain in quarantine for 10 days (can be shortened to 5 days with negative tests).
• Children under 6 years of age must quarantine for 5 days (high-risk) or 14 days (virus variant) upon entry.
• All travellers from virus variant areas must quarantine for 14 days and cannot shorten that time by testing negative.

This is summed up in the following picture by the Federal Government (in German):

Details and an FAQ can also be found in English on the web pages of the Federal Government.

Therefore, under the current rules you can enter Germany as triple-vaccinated adults during your ten hour layover.