[jonah]

Well before that volume of sales, people generally set up a merchant account and use a payment processing gateway like Braintree or Authorize.net instead of one of these intermediated systems. You can then negotiate your rates down below what Shopify+ charges. (You can also configure your own gateway with Shopify. Here is a list of the ones supported in the UK: https://www.shopify.com/payment-gateways/united-kingdom )

It really depends on the needs of your business. Does the hosted solution do everything you need it to? Are you hampered by a lack of features or flexibility? How complex are your product offerings, and content management and marketing needs? What are your growth projections? These types of questions should have a larger influence on your decision than transaction fees.

If you want to build your own site, check out SpreeCommerce (Rails) - https://spreecommerce.com/ and ReactionCommerce (Node/Meteor) - https://reactioncommerce.com/

Don't hesitate to add more detail or ask me more questions here or offline - I deal with this stuff all day.