Kevin1911 – policy may differ now, but when I asked D&A and Vision Express about this a few years ago, they said Oakley UK will only fit Oakley prescription lenses bought at the same time as the frame. I assume that didn't/doesn't apply to frames with inter-changeable lenses though (e.g. Jawbones) because people might wanna buy different/spare sets of lenses.
You could probably get non-Oakley lenses (e.g. Nikon, Transitions) done at any Oakley authorised opticians though. If you want ultra thin/high index lenses anyway, this is probably the better option as they will be able to go thinner/higher index than Oakley offer.