This band around the tail mean "Friendly aircraft, don't shoot them"
Is a part of RAF quick recognition system (as well spinner colour and yellow wings edges). Someone noticed that national markings are clearly visible only from close distance so these markings were introduced.
German planes wear yellow enginie nacles and yellow rudder (sometimes yellow wing tips or band around rear fuselage). On Mediterrainan Theatre German QR markings were painted on white.
Thank you net_sailor, i am in the process of doing a model of Bader's Spitfire that he was shot down in, i have seen two illustrations of it one with the band on and one without and need to know if he had the band on his plane when he was shot down.