What are the UK tax implications if my pension fund is substantial?

A transfer to a QROPS will be what is known as a “benefit crystallisation event”. This may give rise to a tax charge if the amount transferred exceeds the “lifetime allowance” (LTA) – which has been £1.0 million from April 2016 onwards.
If, however, you have registered for “primary protection”, “enhanced protection” or “fixed protection”, then lesser tax charges may apply.
It is worth noting that the benefit of a QROPS is that once a large pension is outside of the UK then any further increases will fall outside the lifetime allowance rules.
In addition, the tax on transferring a large pension out of the UK which is in excess of the LTA is lower than if you take the benefits in the UK. The tax rate on the funds in excess of the LTA is only 25% on a QROPS transfer, as opposed to up to 55% on withdrawal of the funds from the UK scheme.