since you're not in the US you'll want them to tap the holes to bolt the cover on with an M4 thread instead of a 8-32 since I doubt you can even find any american standard thread sizes in UK.
I would assume that it would fine to have them just tap thru the entire block since that's on the outside edges and won't effect anything and just make sure the screw you use is less than the combined thickness of your cover + block.
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