Read the bloody thing before posting! ;)
As for spell check, you can change the dictionary in Firefox to UK, but I don't know about Opera.
I've been finding I've been making a lot of typographical and grammatical errors recently (maybe it's just the advance of years).
Can any of you provide top tips (other than just READ THE BLOODY THING BEFORE POSTING) for lowering the instances of this happening?
If you are worried it is the advancement of years, keeping your brain sharp through mental exercise is not a bad idea. Things like reading a challenging book can go a long ways in making your mind put out more effort than it is use to having to on a regular basis. If you are the type that doesn't read at all (which just from your posting style, I cannot imagine being the case), reading a book will help. If you are the type that reads crappy McNovels (John Grisham, James Patterson, romance novels, typical mystery novels, most things you find on the NY best seller lists, written on a 5th grade reading level for mass consumption), then reading something more substantial will help.
If you are already into reading heavier works, you could try to find something heavier, or take up the task of learning to read those heavier works in their original tongue (now I am just getting ridiculously unreasonable). Attempting to teach yourself Greek and meander through The Odyssey in its original language, or attempting to learn Italian and read Dante in the language in which he wrote, would work all sorts of wonders on a rusty brain. If linguistics is not your thing, then attempting to learn a new musical instrument can also open up vacancies in the brain, letting the maid in to clean out the cobwebs and make the bed, turning what was a dark and empty unused room into a warn an inviting well lit environment for new knowledge to reside.
If we could get more people to give their brain a good work out from time to time during their midlife years, we wouldn't have near as many problems with loss of cognitive ability like dementia in our elderly. Unfortunately, we usual develop our mental habits at a young age, and continue doing the same sort of things for the rest of our lives, not really taking the opportunity to challenge ourselves mentally once we are no longer obligated to through high school or college.