Did a quick check and came up with this link
http://www.cavendishonli...investment/exit-charges
It implies that a re-registration of funds ISA & non-ISA held to another platform (ie the funds are not being sold and repurchased) is likely to be free if moving between the main platforms.
I think the caveat is that the funds are assumed to be held on the rival platform on an execution only basis.
It looks like Cavendish is setting the benchmark, having a nominal cost breakdown of
0.75% fund AMC + 0.20% platform fee + 0.05% broker fee = 1% with the other 0.5% returned
Invesco Perp Income = AMC 1.5%
= 0.50% rebate from cavendish
= 0.25% rebate from BI Select
= 0.25% rebate from H-L
= 0.25% rebate from CommShare**
** If you hold significant funds with them you can get them to increase it. I get 0.375% after I suggested I would move. The reason I've not moved was I also have funds on CoFunds and up until recently they'd all of had to have been sold and the transfer made in cash