Hi Kath and a warm welcome aboard Filipino UK
Congratulations on winning your Fiancee visa
Now that you made your first post I hope you'll share more about your own immigration journey.
You'll find plenty of friendly support here.
In addition to the post highlighted by Rayna I can also say that I contacted UKVI on this very question and here's the reply they sent me:-
So don't be concerned about that.Dear Mr xxxxx
Thank you for your email.
The TB certificate needs to be valid at time of application to show that the applicant is free from TB. It doesn’t need to be renewed before they travel to the UK. As long as it is valid when they apply. We hope this clarifies your concern.
Yours sincerely
Mrs xxxxx
Entry Clearance Assistant
Correspondence Team, Manila Visa Hub
UK Visas and Immigration
Home Office
British Embassy Manila