I can't say I have any single obscure gem to recommend, but I can recommend this book called "The N.W.O.B.H.M Encyclopedia" by Malc Macmillan. The title tells you everything you need to know: it's all here, 800+ pages worth of obscure bands, demos, albums, ep's, everything. I can't believe a human being put so much effort into this huge pile of underground metal, like most of the bands were nothing more than one ep wonders, some of them released only one demo, and this hardcore dude has everything covered in detail. That's insane.

Edit. Seems I made a great discovery after all: the band is proggy metal group Big Daisy and their only single Footprints On The Water/Fever is actually brilliant, and surprisingly not dated at all. The singer/bass player later played also on Wishbone Ash if I recall correctly.

Edit. Edit. Wait, there's more. There's this awesome compilation album called "The Neat Singles Collection", there's multiple parts, and they consist of all the singles released by Neat Records. Now that is one big goldmine if there ever was one, lot of these songs have been long forgotten and rare as balls.

My favourites NWOBHM debut albums are:

Def Leppard - On Through the night (They are no longer Heavy metal, but first 3 albums are killer Heavy metal that defined sounds)
Praying Mantis - Time tell no lies (Probably de best debut album of any NWOBHM, just fantastic and perfect, they followed a more melodic sound after that, but this debut album is something you should listen if you like 80s Heavy metal)
Tygers of pang tang - Wild Cat (They gave us a lot of amazing work in their early days, a lot to enjoy there)