The waiver may be meaningless if the provider has breached the contract.
The "non refundable deposit" may also be illegal if it's a distance-buying situation (you did not visit them to buy the product/service) and it's a mass distribution course that they sell to many people, rather than something personalised or bought in explicitly for you.
Re. the failure to respond to email - see section 3.4 on page 14 of the document here:
"A commercial practice is unfair if:
• it is not professionally diligent"
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/284442/oft1008.pdf
Read all the way through that and see what other regs they may have broken, then write to them and say you demand a refund due to breaches of the Consumer Protection regulations.
Note that the OFT (Office of Fair Trading) has been replaced by the "CMA Board".
Contact your local Citizens Advice centre for legal assistance and backup with enforcing the regulations if needed. Note that you may need to ask specifically to speak to one of their solicitors as the normal volunteers, though helpful, are not always fully knowledgeable on the details of consumer law; they can even be totally wrong..
The site the document above is from, with more information: