Hi,
Letter is for paying biometrics fees and sharing receipt with them, not for doing the biometrics within 5 days. There is link in the letter where you can pay biometrics fees, you don't need to go to London for that, just do it online. Usually biometrics must be done within 30 days of application submission, they will send you a letter asking for Biometrics and then you will have to go to closest VFS office for biometrics.
Regards,
PS: Not a legal advise, this is based on what I understood from the letter.