IIRC their charges are very high.
I had a quick goggle and found a page saying for pensions "There will be an initial product charge of 1.5% of your investment. There will also be an annual product management charge of 1% but this will be waived...." And another for ISAs saying "The total of these charges is therefore a charge of 5% of the initial investment and an ongoing charge that for a typical portfolio will be between 1.6% and 1.9 ..."
When your growth (after inflation) is a few percent a year, this takes a big chunk out of it.
The salesmen will say that their company's skill and knowledge give you such big returns that it compensates for the high charges, but numerous studies find that no fund consistently bears the index, and high charges lead to lower returns.
Interactive Investor charges £10 a month and some Vanguard funds have charges of 0.1% a year with no initial charge. There are other good value companies around but those are examples I use.
In my case I also have shares in particular companies. This adds the risk that you make a bad call, or stick with someone going downhill. I once had shares in RBS
An account at Interactive means I am not tied to any one fund management company. I do everything online.
Best advice is probably to go for low-cost trackers. You will not get the services of a salesman. Sorry, I mean "adviser"
You can diversify by having, say, a UK fund, a European fund, a Far East fund, an International fund. Some people think that since Brexit, UK is bound to lag Europe. So far they have been right. If you think that Healthcare is a coming growth industry, you can invest in a Healthcare fund. No stock picking, so no highly paid guesswork, they just invest in the wide market.
Probably unwise to put everything into one sector.
I generally buy the Income version of funds, there are usually Accumulation or non-distributing ones which are otherwise the same. I set mine to "income reinvestment" but when there have been hard times, I just change it to have the income paid to me. No need to sell holdings or close the account. I also have shares in Investment Trusts, and in a "Fund of Investment Trusts" which has been one of my best.
My main pension has six constituents, which is enough because each has hundreds of holdings.