The AVC provider is now Scottish Widows.
RM's AVC's are 'in-house', so they're designed to offer additional benefits alongside their main scheme. I.e, to fund the tax free lump sum. But they also have the flexibility of free standing AVC's, which are basically the same as a personal pension, and can be transferred. More info: https://www.pensionsadvisoryservice.org ... and-fsavcs
If in doubt, I suggest you contact your SIPP provider for clarification.
Also bear in mind that only the first 25% of your SIPP is guaranteed to be tax free, with the rest being classed as income. Your AVC's may be completely tax free, if under the 25% total pot value.
Plus when the CDC scheme is introduced, we may no longer be able to pay into the current AVC arrangements, therefore transferring may not be the correct move? (obviously your choice).