Sounds like Stripe has a few suggested solutions for this use case aside from multiple plans: adding an invoice item, or using quantities that don't map one-to-one with the quantity of what your customer ordered (in your case, having a base price of £0.50 with the first domain contributing a quantity of 2x).
Yes a free plan with 6 invoice items added via webhook should capture this as a single bill. At the end of the month stripe pings your server for user X what is the bill and you respond with your 6 line items.
https://stripe.com/docs/subscriptions/quantities#dynamic-bil...