The “French” doors you usually find in the UK are not French at all, as you’d probably expect. As you close proper French double doors they fit together by one door having a rounded edge profile and the other door having a cupped edge profile. When they’re properly sized and when the wood is properly seasoned, prepared and painted so that it doesn’t expand or shrink too much, this is an excellently draught-proof arrangement. Naturally it’s getting harder and harder to get proper doors that are properly fitted.
Here in the UK, “French” double door fit together with rebates. The edges where the doors meet have L-shaped profiles. This is nearly impossible to draught-proof.
Fitting locks (deadlocks or sashlocks) to either countries’ arrangments is challenging. Normally a rebated door will have a rebated lock. The lock will either be specially shaped or will have an adapter such that the bolts and keeps (boltholes) can still be placed centrally, across the rebate.
A lazy or ignorant fitter might be tempted to fit the lock off centre so that they don’t have to deal with the step shape of the rebate. Unfortunately this means that the bolt keeps will find themselves in the thin and fragile sticking out part of the other door. And the lock case will be too close to the face of its door and easily broken out and opened.
Even when fitted properly rebated doors won’t be secured particularly well by a deadlock or sash lock. Most deadlocks and sash locks, for example, lose their BS (British Standard) rating when used between rebated double doors.
You really need to fit mortised bolts as well. Mortised means that the bolts are inside the door rather than screwed to its face. They are also known as rack bolt or star-key bolts because a star shaped key that goes into a small hole is used to wind the bolt in or out.