Actually, it would be the Royal Air Force that (hopefully) came to get you. Your odds of survival depended on one of your fellow pilots radioing in that they had seen a 'chute over the channel, or you managing to radio similar, at which point Coastal Command could coordinate your rescue with the Royal Air Force Maritime Branch.

The Maritime Branch, who had the rather excellent motto "The Sea Shall Not Have Them", operated a fleet of several hundred power boats and motor launches around the coast of the UK and overseas territories with the explicit purpose of rescuing downed aircrew. During the Battle of Britain, you're likely to have been picked up in one of these, a High Speed Launch Type 2.

Expected Survival Time in Cold Water

Water Temperature Exhaustion or Unconscious Unhurt

60–70° F (16–21° C) 2–7 hours 2–40 hours

50–60° F (10–16° C) 1–2 hours 1–6 hours

40–50° F (4–10° C) 30–60 minutes 1–3 hours

32.5–40° F (0–4° C) 15–30 minutes 30–90 minutes

https://www.iwm.org.uk/history/listen-to-raf-pilots-tell-the-story-of-the-battle-of-britain

First of all, planes did not have ejection seats like you see today. In order to escape a severely damaged plane, you would have to open the canopy and bail (jump) out. This was only done when the plane was severely damaged to the point that it was unflyable or on fire.

Many pilots decided instead to ditch their planes. Ditching is attempting to crash land on top of water and hopefully scramble out of the plane before it sunk. This was actually safer than bailing out.

I’m not sure you’re going to find an exact percentage, but from certain information you can infer that survival was unlikely.

In 1940 the RAF did not have a dedicated Search and rescue service. It wouldn’t be until 1941 that the British established a dedicated rescue service under coastal command. Due largely to the losses incurred during the Battle of Britain. Instead because of the high traffic in the channel they mostly depended on a ship or boat to spot the down airman and rescue them.

The temperature of the channel meant that a pilot in the warmest time of year would have an expected survival rate of 40 hours. The average temp of the channel is in the low 50-degree range.

The RAF lost 540 airmen during the Battle of Britain. It’s estimated that more than 200 died while waiting to be rescued after successfully bailing out or ditching their plane in the channel.