A legal opinion from the parliament says, "As long as the UK is a member state of the Union...it is under an obligation to hold elections to the European Parliament. A failure to hold such elections would entail a breach by the UK of its obligations under the Treaties".
According to the European Parliament’s legal service, that means if Britain is still a member of the union at "a time close to the elections" it would have to hold them, or be in breach of the treaties.
The next elections for the European Parliament are due at the end of May. That would also mean that British citizens living in Sweden would get to vote.
The British government wants to renegotiate the withdrawal agreement from the union, but the EU has so far refused. The British parliament is still struggling with an alternative to Prime Minister Theresa May’s Brexit deal, which was voted down by a record majority in the House of Commons in the middle of last month.