I think that should work Dave, as once it has measured and stored the values for the antenna, it should simply use those settings. Being in the loft, nothing is going to change, is it?
Actually, why not simply make up a matching network to match the impedance consisting of capacitors and indicators and leave that there at the feed-point, no ATU needed. To find what you need to match to (from is the 50-ohm coax and rig) put an antenna analyser (borrow from a mate or your local radio club if you don’t have one yourself) on the feed point and note down the readings. There are on-line sites that will give you the circuit you need to build - maximum of three or four components and you’ve done what the ATU would be doing.
73 Ed.