Funds these days, in the UK, tend to mean unit trusts (on their way out fortunately) and oeics. Anyway so when are funds costly? Having given it a little thought, I would suggest, when there is a high annual management (over 1 percent ?) for a straight forward fund of UK shares.
High annual management charges (amc) also referred to as ter, when extra costs are taken into account, bite when you have large holdings. One percent of 100,000 pound is 1,000 pounds. If the amc was only 0.5 percent you'd save 500 pounds per year.
Now there are occasions where OEICs purchased within an ISA may be reasonable value. IE when the fund offers some sort of specialisation. Eg investing in particular parts of the world; bond funds where income tax relief cuts in; more complex funds offering enhanced income through various techniques . . . so it may be wrong to neglect oeics where they are tailored to your more specific needs.
Large Investment Trusts may well help reduce costs for large holdings of straight forward UK or general international trusts. Of course you could by UK shares yourself and pay no one to manage them cutting down on amc all together, bearing in mind though, you ought to buy at least 1,000 per transaction to keep dealing charges down, you would want a holding of 5 or more for at least some degree of diversification (so you don't get destroyed if a particular holding goes up in smoke) and you would be well advised to pay more attention on how your shares are doing.
Just a few thoughts, any other ideas appreciated.