Obvious Pros - you get a lovely house you couldn't otherwise afford.
Obvious Cons - Depending on how busy the road is, obviously safety and noise.
I think moving to the country from London is a fab idea (although I am biased!)
When I was younger we moved from a city to a small countryside town and my parents never regretted it.
You could keep looking for a home in the country that isn't on such a busy road?
I would agree with Specialsubject and MyWaterButtisEmpty (someone please teach me how to tag haha)
If you are going to make such a big move (and investment) to move to the country, you will want to enjoy all the benefits that come with it.
Where I grew up (Petersfield) I was able to walk to school and back - even to the town and back on my own and safely. And surrounded by lovely countryside - although Petersfield is a bit more developed now!
There are however things you can do to help with the acoustics. Are the windows double glazed? You might need to re-do the windows for better acoustics and this will obviously be at extra cost.
You can get a quote for windows online to help figure out a rough cost here: www.doubleglazingontheweb.co.uk/double-glazing-quote/
I've used it and it's fairly simple to do.