I've used HL for several years now to manage a SIPP. Although it's UK - sourced pension funds invested, I've not been UK resident since establishing the SIPP with HL (I opened it living in Greece, now living in Ireland), and this has posed no problems. (For example, they will produce statements in a Greek/Irish tax year format, 1/1 to 31/12, rather than the UK tax year of 6/4 to 5/4 without any hassle, or charges).
I imagine that the trading platform and admin. functions will be much the same whether it's straight investment or a SIPP, and they are very efficient. Dealing is prompt, all contract notes etc are electronic, with very little in the way of paper coming through the letterbox. My account has good historical/analytical data for available funds (I'm wholly in funds, so can't comment on equities, but I assume it will be similar), with occasional market overviews thrown in.
As far as I'm concerned, they are very fit for purpose and I have no incentive to move after having used them for some time.
Paul