It so;unds like you've already worked out that the ISA and the investment are two different things, so I'll not labour that point.
If you go with a self select ISA from Hargreaves Lansdowne or similar, then you're going to have a huge range of funds to choose from, including tracker funds, and actively managed funds. There will be a fee for each fund you're invested in, and also a management charge. Some trackers are cheaper than others - the last time I looked, the Fidelity Moneybuilder one was cheapest, but that was a while ago. The Vanguard LifeStrategy funds are also seen as being a good price.
Alternatively, you could go with someone like Virgin Money, who will sell you their own fund - no management charges, but the fund charge is high compared to others.
Someone mentioned Nutmeg. They are an investment management company, who will take your money and invest it in what they think will do well - not usually a tracker. Their fees are higher than a tracker, but lower than an IFA.
The option you want really depends on what risk level you want, how much you're investing, and whether you want to actively manage it yourself, or hand the money over and forget about it.