UL products only have contract boundaries if the charges are completely reviewable so that they fully reflect the inherent risks. The whole issue about whether certain UK UL contracts do or do not have contract boundaries is one which is not particularly well resolved yet, so if you are looking for an example I would stick with the one that has been raised earlier in this thread, ie a contract with reviewable premiums such as a particular design of whole life assurance or term assurance where the premium rates are fully reviewable every ten years, say.
Contract boundaries means that future premiums payable after that boundary are not included in the BEL. Therefore the concept has no meaning for single premium business, on which there are no future premiums. So immediate annuities, which are single premium, do not have a contract boundary. Deferred annuities tend to be written on a conventional without profits basis, with fixed premiums, and hence there are no contract boundaries as the premiums cannot be reviewed and the product cannot be terminated unilaterally by the company.