Some other thoughts for you.
1. As you are very sensibly targeting only modest rates of return from your ISA, the issue of costs becomes very relevant. If you invest in actively managed unit trusts then you can expect to pay management charges of between 0.8% and 1.75% per annum. If markets are only delivering 5 - 7% per annum returns, these costs will eat into your returns significantly. Costs on investment trusts do tend to be lower. However, I have given up investing in stocks and shares via actively managed trusts because the average returns over time are no better than the performance of the underlying indices. Yes, someone can always point to a series of actively managed funds that beat the index over the past one or three or five years. This is irrelevant. The issue for investors is, can you select those funds that will beat their indices in the future? If you cannot, and I certainly cannot, then I think you are better to go for index trackers where the annual fees are of the order of 0.25% - 0.5% per year. Also consider exchange traded funds, such as ishares. These have very low annual and dealing costs.
2. I would not personally put all my investments in one asset class. At your age, and given your investment goals, a mix of 50% bonds and 50% equities would be sensible, or even 60:40. The equity funds should also be spread across different markets - say UK, USA, Europe and emerging markets. That will tend to increase your returns and reduce their volatility.
If you have time, you might find Malkiel's book, "A random walk down Wall Street" interesting and stimulating.
Good luck with it.
