Halifax automatic dividend reinvestment

Yes, the accumulation fund is certainly the simplest method for anyone with an ISA.

However, remember that Halifax will want to take their £12.50 admin fee every year, so if you are not getting paid any dividends you need to have some money that's not actually invested sitting around to pay the annual fee, otherwise Halifax would have no choice but to sell some shares in your fund to create some cash for them to take as their fee, and would charge you a fee for doing that, so you would get extra fees.

If you had the "income" class instead you would be generating cash every dividend date so would always have easily enough to pay the ongoing fees (because for example £5k invested would be paying you over £50 a year and the fees are only £12.50 a year) but then your problem would eventually be too much money sitting on the sidelines. If you have it auto-reinvested you have to pay a high 2% fee to do that, which is undesirable, and auto reinvesting all the divs means that no cash is actually left over to pay the annual fee which then defeats the point of using income units in the first place.

So, when you invest your lump sum, make sure you leave a bit of cash left over so they can take their fee from that if you don't bother to log into the account and add new money - and then you won't need to pay fees to sell shares to get cash for fees.