I need some creative (but legal) help to minimize FX rates for a domestic transfer to my Singapore based OCBC GBP account.?

I have SGD in a DBS and UOB bank account. I need to transfer a large sum to a GBP account held with OCBC to pay off my UK mortgage. I'm hoping to save on FX by using Transferwise or InstaReM but it doesn't seem to be possible to send foreign currency domestically.
Is there a way to make the transfer by prepaying the Transferwise debit card or using Multicurrency accounts without incurring large fees? Other suggestions welcome. Thanks!