The Medway Handyman wrote:

One old boy came up with a 'solution'. Put several elbows in the
waste pipe, to take it down to floor level, under the unit, back up
the other side & reconnect to the waste - effectively forming a huge
'U' in the waste pipe.
I tried to explain that it wouldn't work, the huge 'U' would always
be full of water & the sink wouldn't empty properly.

That would work, it's just a giant trap. Any new incoming water raises the
level and displaces water 'over the edge' into the downstream pipework.
Like a WC bowl - they can cope with rushing water.

On the other hand, it would be highly vulnerable to blockages, any grit or
fibre which settled in that section would stay there, gradually gumming
together with hair, fat & fishbones. Ask the old boy to promise that he'll
dismantle it and flush it with caustic soda once a month, and it might be
viable.