Try Jenkins Steel Online perhaps (mainly steel but not only). They certainly are in business. They quote a price for each size with "free P&P", although clearly that just means it's already been included in the price.
I reverse engineered their pricing the best I could and in their case it looks like a different flat charge for each section plus the cost of the steel. For the 300mm length of 110mm black round EN3a I bought, it worked out about £18 regardless of length plus £1.30-1.40 per kg (inc vat). Total about £46. Sounds a lot for carriage but you're talking about dense stuff. Their ebay shop is more expensive, presumably due to the additional fees ebay charge.
Murray