For anyone who first claimed the remittance basis in 2014/15, the deadline for making the election is 5 April 2019.
Although the claim to the remittance basis is an annual decision, the foreign loss election is irrevocable and, if not made, relief on foreign losses is gone forever. If made, the election means that foreign losses remain allowable in a tax year in which the remittance basis applies. The losses are relieved in the following order:
Foreign chargeable gains remitted in the tax year
Foreign chargeable gains in the year, not remitted to the UK
UK chargeable gains
It is not possible to cover all the potential permutations of these rules here but if you believe you might be affected it is important to obtain professional advice, not only on the loss relief itself but also on the administrative requirements, and take any beneficial action ahead of the deadline.