When choosing an investing platform I'd prefer to keep it simple and have accounts with just one company. I'm happy that my investments are safe, so grouping things together makes my life easier and often means economies of scale, ie lower fees.
What are the important factors?
In fact, lower fees are probably my primary concern. I do expect some fees, since I know they're providing me with a valuable service, but I don't want to pay over the top.
The service is important. I want a good range of investments to be available, and I want to get a reply to my questions.
Stability is also rather critical. I don't want to be doing business with someone who is here short-term and then selling my business on next year - I'm here for the long haul and don't want to be messed about.
All Purpose Platforms
If I need a platform to host the normal accounts - ISA, SIPP, and GIA - then these are my picks.
Starting out: Vanguard or Hargreaves Lansdown
From £10k invested: Vanguard
From £100k invested: Interactive Investor
Here's my reasoning for these choices. Vanguard and II are both decent platforms. Vanguard work out cheaper when you have a smaller pot invested, with their 0.15% annual platform charge, and from about the £100k mark II with their flat monthly subscription fee become cheaper than the percentage charge.
For small pots below about £10k, HL may work out a bit cheaper than Vanguard. They charge 0.45% annually, while Vanguard have a minimum fee of £4 per month. So you could save a little bit by going with HL initially.
But if you're going to reach the region of £10k pretty quickly, you may not think it worthwhile to chop and change, so you may prefer to set up with Vanguard in the first place.
Then there's InvestEngine, the plucky little startup offering accounts free of platform fees. For me, this fails to pass muster on the aforementioned stability test. It's a loss-making startup, so as it stands offering accounts for free isn't sustainable. There is a real admin overhead, particularly in offering SIPPs, so it remains to be seen how these guys will work out. Will they introduce fees? Will they get out of this market and sell your business on? Will they squeeze a profit from the buy and sell prices when you trade? But if you want to use them in the meantime, you'd save a few quid.
Note that if you want this SIPP to receive your workplace pension contributions, that rules out Vanguard, who only accept these contributions if you're one of the company directors. Annoying.
LISAs
For lifetime ISAs, there's a much more limited market. The regular faces like Vanguard and Interactive Investor don't offer them.
Perhaps this is because the contribution limit of £4k per year, along with the limited band of ages during which you can contrbute, means they stay as relatively small accounts - and small accounts are hard for platforms to make a profit on.
The two familiar-name platforms offering them are:
Hargreaves Lansdown - fees 0.25% with a cap of £45. Dealing funds: no charge.
AJ Bell - fees 0.25% but with no cap on fund fees. Dealing funds: £1.50.
HL edges it for the win.
Junior ISAs and SIPPs
Fidelity don't charge a platform fee on Junior accounts, though they charge £1.50 dealing fee for regular monthly trades and £7.50 for other trades.
Vanguard charge 0.15% annual platform fee then no trading charges. Although they're in the process of introducing a minimum £4/month fee, it doesn't appear that this will apply to junior accounts.
If you have under £12,000 in there, and are investing on a monthly basis, Vanguard are cheaper.
If you have over £12,000, or are making a one-off investment for your kids, or are contributing less than monthly, then Fidelity wins.
Freebies
Another factor which can shift the balance if you're comparing the costs is whether they offer any freebies. Some platforms like to entice new customers to join or transfer their business over with some sort of introductory offer. I wouldn't let this sway your decision unduly, but if you are going to a particular platform anyway then do be sure to check out what offers you quality for.
Vanguard never do this.
Interactive Investor almost always have an offer on the go. If you need a referral code, here's mine. Full disclosure, if you use it, I get a reward too.