The British ruled the seas in the last half of the last millenia. What was the state of the British navy before then?

Firstly, it’s worth noting that ‘Britain’ only existed from 1707 onwards, when the Act of Union united England and Scotland. So really, your question is about the English navy, not the British.

In the Middle Ages, there was no English navy as such. The English crown could at best muster a small handful of warships. When needed, English merchant vessels were called into service. This was the case in the naval battle of Sluys in 1340. Of the 150 strong English fleet, only 3 were owned by the Crown. The rest were privately owned cogs. They were still very effective. Two English ships were lost during the battle. The French lost 190!

Even during the Tudor period, there was no official ‘Royal Navy’. In 1588, the English fleet that defeated the Armada was based around a core of 34 top of the line warships, thanks to the efforts of the naval reformer (and slave trader) John Hawkins. However, the bulk of the English fleet - some 160 additional vessels - were armed merchantmen owned by private individuals.

It wasn’t until the 17th century that England began to sow the seeds of its later naval supremacy. Reforms brought in during the commonwealth period improved naval tactics and professionalism. The dedication and talent of Samuel Pepys laid the foundations for the organisation of the new ‘Royal Navy’. There were still setbacks, such as the humiliation of the raid on the Medway in 1667. But by 1707, when the Act of Union was signed, Great Britain was already well on its way to ‘ruling the waves’.