What car is it for?
Does it already have speakers?
At £50 20wx4 would be enough frankly. Quality doesn't come down to wattage and the bigger the number the less chance of affording speakers that'll handle the power without giving up the moment you turn up the volume.
My Civic was fitted with 35wx4 as standard for what was an £18k car new. It's only 10years ago that 20wx4 was the norm!
Suggestions:
Pixmania always have decent prices:
This Tokai has USB/MP3 for £36 inc VAT and you'll see on the site they've loads of deals for speakers etc.
This iRandom is similar but is 40wx4 at £45.99.
I'd be tempted by
this Blaupunkt and go with SD cards instead, is £10 more gonna break the bank? (£60)