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Classicedge.ca sells scuttles and their instruction is to fill it with hot tap water, then dump it and replace with boiling water.
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A well made scuttle should be able to take boiling water.

That pretty much nails it. Mass and water jacket volume are the biggest determining factors of the scuttle. Start with pre-heating the scuttle and you are good for the whole shave

That being said, I'd go with the larger one which has a higher water capacity

That's from tap water alone. None of them work well from just tap water. If you use boiling water to pre-heat any scuttle you will be fine.

How about this? In the UK too.
http://stevewoodheadceramics.co.uk/index.php/shaving-scuttle-shaving-mug-shaving-bowl/shaving-scuttle.html