LONDON (Reuters) - Two British men who tried to smuggle 18 Albanians into England by inflatable boat in May and had to be rescued in choppy waters were each sentenced to over four years in prison on Friday, Britain's state prosecution service said.

News of the rare attempt at a clandestine sea crossing was seized upon by media and some politicians who were campaigning at the time for Britain to leave the European Union as evidence of migratory pressures and the need for tighter border control.

Thousands of refugees and migrants have tried to illegally enter the country in recent years, mostly by stowing away on trucks or trains bound for England from France, although the numbers are dwarfed by flows of people in the Mediterranean.

The two smugglers, Robert Stilwell and Mark Stribling, were the only people on board the boat who were wearing life jackets. Passengers had paid 6,000 euros ($6,700) each to make the trip, which British authorities said had nearly cost them their lives.

The boat's battery failed shortly after it left France and water started flooding in, leaving those on board to frantically try to bail it out as fast as they could.

Local lifeboats were launched, along with a Border Force vessel and a search-and-rescue helicopter. The rescue took around an hour and a half in rough waters with poor visibility. One woman had to be treated for symptoms of hypothermia.

"We may well have seen a tragedy just off the British coastline," said Kris Venkatasami, deputy chief prosecutor for the southeast of England.

Both smugglers had pleaded guilty at Maidstone Crown Court to conspiring to assist illegal immigration. Stilwell was sentenced to four years and four months, while Stribling received four years and eight months.

Over a million people entered Europe in 2015, mostly fleeing war or poverty in the Middle East and Africa, with hundreds of thousands risking their lives to cross the sea from Turkey to the Greek islands or from Libya to Italy.

Britain is not part of the border control-free Schengen Area and has been largely shielded from the impact of the crisis, but media coverage of attempts by migrants to find routes to Britain from Calais in northern France played a part in the EU debate.

The Calais issue played into the hands of those like anti-EU party UKIP, who argued that the country was taking in too many foreigners and should regain full control of its immigration policy by quitting the bloc.

Britain voted to leave the EU in a referendum on June 23.

(Reporting by Estelle Shirbon; editing by Stephen Addison)