There is ash in various concentrations over the most of the UK at present. If any of these concentrations are assessed to be high, the CAA will issue a NOTAM creating a No Fly Zone for the relevant coordinates and levels. At present the common division is SFC to FL 200 and FL200 to FL350. A No Fly Zone means exactly that: you may not fly in it, VFR or IFR. To do so will render you liable to prosecution for a breach of the ANO.
At the time of writing this, there is a No Fly Zone covering the most of Wales, the Cotswolds, the London area and Kent until 0600Z. This zone has moved SE over the last 24 hrs, so it might continue to do the same. Your VFR positioning flight may well be OK unless it lies in these zones.