No one really knows when a sale will be along. They come and go and there's no 'official' sale on just now but I do recommend regularly looking at BA's home page under offers and deals where you will see 'Find our cheapest flights'. Fares there fluctuate a fair bit. Personally, September is a long way out and I would hold off unless you see a price your happy with. What prices are you seeing just now?

Ex-EU fares (starting in say Dublin or Amsterdam for example) can also be good of your looking to fly in a premium cabin and starting in Inverness can also be fruitful. More in the BA Tier Point thread and there is a Premium Fare thread too as well as historical fares here too.

BA Sale Fares 2018 « Continental Club Blog

In the meantime welcome to Flyertalk and the BA Board melris.