Aye Steve,
With the old BT45's you used to get some uneven wear patterns on the front tyres. I suspect because they used two different rubber compounds. The harder of the two compounds for the central tread and the softer for the two outer areas of tread.
However, because all the SRX's that I have used/owned required new tyres, I have always fitted Avon RoadRiders and more recently Avon RoadRiders2, as they are readily available and a more reasonable price than the Bridgestone Battleaxe from my local supplier. I have had wee slides from the rear in heavy rain, with low temperatures, when pushing it a bit too much. Mileage wise, the Avon's normally managed about 7,000 to 8,000 miles, with under 2mm of tread remaining and my pottering!
But having never ridden an SRX with either BT45's or BT46's, I cannot comment. Many of the people I know riding Guzzi's, Commandos etc are still using RoadRider2, without complaint.
If you prefer BT46's then 100/80x17 and 120/80x18 are available on eBay (2023-05-12) for £86-63 and £124-65 respectively and "Free P&P".
100/80x17 on eBay Lot: 295515683127 and 120/80x18 on eBay Lot: 295529366125
Both advertised by "Dennis Winters", Beverley. So you would like to think they would be "Reet!"
Good health, Bill