what size boiler should i be looking at?

[Mike Jackson]

That's exactly what the excess is for. Otherwise little Johnny is washing his hands under the hot tap in the kitchen sink. Dad has a slash in the downstairs loo and flushes it. As the stop tap is in the downstairs loo the cistern will draw cold water and reduce the flow to the boiler. The flow to the boiler reduces to 4 l/min and, briefly, before the boiler modulated down the water is heated to 80°C. Little Johhny ends up in A&E with full thickness burns to his hands.

You should be able to operate another outlet in the property without it reducing the flow through the boiler.