Excellent accurate replicas of flying helmets and masks can be bought from Stephen Silburn of Sefton clothing. He trades as Izavailable on eBay and can be contacted through there. His work is top notch and can be seen in Hendon and Duxford amongst other places he also supplies film companies. I am in no way associated with Mr Silburn, other than being a happy owner of his B type helmet and D mask also a C type and E* mask He can also repair original items as he has worked on some of my original helmets. Whilst not inexpensive his work I feel is good value and will last a lifetime. Better than risking original very expensive items.

For battle of Britain you will also need a microphone for your mask, originals are rare and very expensive £500-£1000 if bought from specialist dealers. Replicas are available and come up from time to time on eBay. You will also need goggles. I do not know of any replica goggles that are correct for BoB period so you will have to get an original pair. There are several patterns that are correct for the period but my personal favourite is the MkIIIa a good pair currently costs around £250-300.
Hope some of this is of help and has not put you off!
jdh