Fire risk
Lets look at the risk analysis then.
I forget the precise figures but a five layer pro racing suit with full underwear, balaclava, gloves and helmet gives the wearer about 60 secs to scarper.
A single layer baggy gro-bag over your M&S shreds is probably worth a couple of seconds if you're lucky.
Against total heli hours flown, how many pilots have been singed where a nomex suit would have helped, not too many I'd wager.
Whiff of smoke and you hit the ground running, serious blaze is usualy post stoof where unfortunately you and the machine are usualy beyond repair.
On a lighter note I don't know of many VIP pax who'd be wildly impressed with a 'professional' flying suit. I'd have a word in your taylor's shell like if your suits are as smart as an overall.