Now I'm confused. I understood you wanted a backbox you could screw to the wall, feed the flex in at the top & out at the bottom and onwards to the fan. You don't have to have a metal backbox set in the wall, a plastic surface box is perfectly acceptable. The glands are to provide strain relief to the flex ( you wouldn't usually use them with installation cable since the cables are held in place by clips or plaster)- you remove the knockouts top and bottom of the backbox, insert the gland and then thread the cable through the gland.
In all this, we've all ignored the fact that you're in a bathroom so zones apply- your new FCU would have to be at least 600mm from the bath/shower OR more than 2250 above floor level
O.b.n.d. Sorry — I didn't know that there were such things as plastic surface boxes, to back the switch pattress that you alerted me to. If these back-boxes are of universal size, I suppose the problem would be solved. I shall now investigate, and decide whether Terry's solution, or yours, is the way to go. Thanks very much, friend, and to all who have contributed. Well, this is a great relief to me! It's surprising how ignorant one can be.
A.W.