A Thomson Airways Boeing 757-200, registration G-OOBF performing flight BY-6133 from Las Palmas,CI (Spain) to Bristol,EN (UK) with 220 people on board, was enroute at FL380 about 240nm southsouthwest of Porto (Portugal) when the crew initiated a rapid descent to FL100 due to cracks in the left hand forward windshield. The aircraft diverted to Porto for a safe landing about 50 minutes after leaving FL380. A replacement Boeing 757-200 registration G-OOBB positioned from London Gatwick to Porto, resumed the flight and reached Bristol with a delay of 7:15 hours.