I thought Rustins did a water-based grain filler, but I think I was wrong. You can get Timbermate in the UK via eBay, but it's around £50 for a small tin, so is probably individually shipped from Australia by the eBay seller, possibly to order. The standard Rustins grain filler needs to be thinned with white spirit before use. I've got some Rustins mahogany grain filler for my Tele refinish but I'm spraying, not staining over that. You could look for a water-based grain filler, but don't get them confused with water-based wood fillers, which are coarser and won't do the job at all well.

However the Manchester Guitar Tech uses Rustins for grain filling and in the comments section on this page, states that it takes up stains well. https://www.manchesterguitartech.co....lecaster-body/

A little goes a long way, as you need to scrape and push it into the pores, and minimise the amount you leave on the surface to sand off. So don't buy a huge tin. A 230g one should suffice. Don't wait too long to sand and 320 grit should be fine for sanding it smooth without taking too much off again. Probably best to allow to do the grain filling twice.

If in any doubt, try the filler and stain on a bit of scrap wood first.