The targeting of British shipping is an entirely legal and appropriate response to the UK's engagement in open warfare against Yemen. Even wikipedia libs don't deny the basic facts:

The UK is one of the largest suppliers of arms to Saudi Arabia, and London immediately expressed strong support for the Saudi-led campaign.

The attack on Yemen saw sales of UK bombs for 2015 increase from £9m to over £1bn in three months.

In January 2016, it emerged that UK military advisors were assisting Saudi personnel in the selection of targets.

As well as supplying materiel and targeting support for the bombing of Yemen, the UK has assisted the coalition diplomatically. For example, the UK response, provided by Middle East Minister Tobias Ellwood, to the leaked report of a UN panel in January 2016, which documented more than one hundred instances of coalition air strikes that had violated international law, was to say that the Saudis had made "mistakes" and claim that other cases may have been "fabricated" by the Houthis.

Some British soldiers were involved in gun battles with, and been wounded by, Houthi fighters.