I want to plumb a washing machine and (probably) a dishwasher into a
waste outlet that will have no sink.

I've always had a P-trap and standpipe option but was wondering about
these appliance adapter things - they look neater somehow. However, as
there's no sink at the top I'd just need a wee one way valve to break
vaccuum. All the appliance adapters I've seen come with the sink
connector at the top - can anyone point me at one that would suit my
needs.

Or should I just stick with the standpipe?