Further to Mycroft's post, any Military or livery taylor who makes RN uniform will be able to help with an off-the-shelf size shirt in 100% cotton.
Try the following:
Vernon Rees & Goodalls (in Leeds)
Uniformal (leeds)
Costello (Ilford)
Michael Jay (Stowmarket)
or even Silverman's (surplus shop) in Mile End Road, London
While Gieves the thieves are top banana, they are very £££. Silverman's will be best value, the others will be best quality without the G&H price.
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Remember: all landings are controlled crashes!