I've not used mytyres or blackcircles.om, but have found their sites useful for tyre info, prices etc.
Most local tyre dealers will be as competitive, if not a little cheaper.
When you select your location on these sites it will usually give you a
garage, a postcode etc. If you recognise the area, this will often give
you a good clue who the dealer actually is. Ring them up & get a
quote, then compare it to mytyres/black circle etc. If the web is
cheaper, tell them where you got your quote & crash, the price will
come down!!
Any probs, you've got a local supplier who had all your cash, not just a commision, if they give good service you'll go back.
Just my thoughts & experience thus far!
Dave