This isn't a simple question as there are different sorts of endowment policies. With-profit policies do offer a guaranteed sum assured plus annual bonuses which when added, usually each year, can't be taken off the value.There is also, potentially, a terminal bonus paid at the end (maturity).
If you have a unit-linked policy these bonuses don't apply, there are unit-linked with-profit policies though, just to confuse things. Unit linked policies buy units each month from the funds you have selected, these policies are almost directly linked to the stock market.
Unit linked policies sound a poor investment, but think of it this way. When the stock market is low in general you buy more units for your money, then hopefully as the stock market rises over the next few years their value goes up.
You have 5 years to go, I would definitely keep them.
Don't confuse the actual investment funds with the Insurance Companies themselves. Funds, especially with-profit funds, do have significant legal and tax advantages.
Another complex area I'm afraid. Always refer to an IFA for further advice, and a review of your finances would be a good idea in any case.
After the mortgage fiasco of the last few years I think endowment policies could well return to favour, although I acknowledge costs will have to come down.
Good luck.