You generally shouldn't use the word Director in a UK job title, unless they are. Nothing to stop you, and no need to register them with Companies House, but ...
It should be noted that only actual directors appointed by the Company
and registered as such at Companies House should be called directors.
To give a job title to an employee such as “Director of Sales” when
that person is not actually a director could result in the company
being liable to third parties who reasonably assumed that the employee
was an actual director and acted on that assumption to their
Source: (UK) Association of Convenience Stores (legal advice pages)
In other words, it gives the (perhaps false) impression that the "Director" has the power to sign on behalf of the company for contracts etc.
So you need alternative names for "Technical Director and an interim Sales & Marketing Director".
If the culture of the company allows, you could use fun names ("Chief Magician", "Del Boy" etc).
Otherwise, just change Director to Manager (even if they don't manage anybody yet), or for a transatlantic sound they could be Vice-Presidents.
Or Chief Engineer / Chief of Engineering, Chief of Marketing. Or Head (Head of Marketing).
Personally I'd go for the Manager names, very conventional for a UK company (even if it does sound a bit below what they currently do).