Hi Akh83,
this is a difficult question, and I guess it depends a lot on whom you could be working with to prepare the grant proposal and the discipline specific regulations and acceptance rate at the finding body you're aiming at.
I could have not afforded a PhD without some external funds, and I managed to submit a proposal and get it accepted before starting my PhD. I was working on a short-term contract as assistant in a research dep already, and my boss there helped me both to write the proposal and by giving his name as applicant!
If you're supervisor is willing to help you in the grant preparation and give his/her name this would surely help a lot. On the other hand, your project sounds rather applied, here in CH for applied projects you usually have to find a industrial partner willing to contribute to the costs...but it may be different in UK.
Good luck!