The Plus option allows international dealing and you can have a trading reserve account, this allows a cash account for funding trades and you can subject to approval borrow against your portfolio.
The vanilla account pays for non ISA trades from your HSBC current account, subject to a fairly small limit, default is £10,200. In practice I have had the regular account, the low £10,200 limit is a bit silly but if you are selling a larger holding and then repurchasing a similar amount its not. problem. When I have had a larger sum, then I added into the bank account a few days before and telephoned for temporary authorisation for the trade and it was OK.
Doesn't matter with respect to the ISA, as you have to fund it with the annual limit each year. The Premier banking works well and for me that is the major attraction. I will be sending some more money to Invest Direct with a view to the Jade status for banking.
HL is very good for shares and ETFs etc but the Beaufort Securities* issue has focussed my mind on having multiple brokers....there is transfer in deal with Interactive Investors and I tempted to take advantage of the transfer in deal to make them an additional broker. Its offering unto £2000 as an incentive but I will only transfer a smaller part of my HL accounts for the £500 incentive, for each of myself and Mrs HS.
Halifax Share dealing is OK and I still use them, but Hargreaves and HSBC are both easier.
If you are not aware of the Beaufort Securities issue, it was a failed broker recently and it showed that ring fenced assets could be seized by an administrator, the FCA cover is £50k per investor. The eventual result was not as bad as it first appeared, but if you wanted full cover then as a couple you would need a lot of broker accounts, personally we would be in well into double figures, not very practical.