Thanks Wojtek - and now I know why they sometimes appear !
Mae Wests. These were made from a rubberised canvas fabric, and were a beige / buff colour. By the later stages of the BoB, some piolts 'painted' their own vests in a yellow dope, or the yellow paint used in roundels. This eventually cracked, and there are a number of photographs showing this. Later in the war, the life jackets were made of a yellow cloth, although not particularly bright or dense in colour. The 'Mae Wests' used in the BoB movie were 1950's / 1960s issue, which looked similar to WW2 vintage, although shorter, but were 'good enough' for the purposes of the movie.
Beaufighter. As this aircraft type is known to have been operational, even in small numbers, during the Battle, and is listed in the BoB Bits lists, it is elligible for the GB. Actual usage, squadrons, codes etc will need to be found by the individual, so yes Jamie, looks like you're fine there. The Squadron's code letters at the time were indeed FK - if they were painted on the aircraft. There is a possibility that the aircraft did not carry code letters at that time.
Antoni is quite correct in his description of the colour scheme for the time, and I have a colour profile (not 219 Sqn) showong this scheme should you need it.
Exhaust colours. The exhaust stacks on aircraft such as the Hurricane and Spitfire were made of steel, and were affected by heat and the elements in the same way as your car exhaust would be. The reddish brown colour is a combination of heat and rust, and the ends of each 'tube', where the hot gasses would exit, were generally also tinged by heat, to a bluish grey colour, and also by the exhaust smoke which, given the engine was 'healthy', and not running rich all the time, would induce a grey 'halo' around the 'muzzle' of each bore.
I normally paint these parts in a dark metallic colour, followed by a layered dry brushing of red brown and an orange shade, depending on the effect required.