Hi Lee,
I suppose a lot depends upon what your pre vat prices were in the first place. If most of your clients thought you were under priced (as happened to me a few years ago) then the added vat may bring your prices up to a more realistic level, in their eyes.
However if your prices are near the local maximum or above, then adding the vat on, may well put a lot of people off. With your loyal repeat clients, I would think that most would just accept the higher price, knowing that they will get value for money anyway.
Three years ago I put up my charges 50% in one go, and rather than losing work I got busier, could have kicked myself, I should have done it earlier. Ive increased them a further 10% since and are due for another 10 - 15% now.
If I registered for vat. my next increase would have to be 27.5% to 32.5% in order to not absorb any vat myself. Would I be confident enough to raise my prices that much?

Dave.