That's the thing, I'm not sure whether there's much difference between the super cheapos (<£40) and mid cheapos (~£100). I've found this teardown:

EUMAX digital ultrasonic cleaner - teardown and review - Page 1

did some research and found that the company is Chinese, but they do offer the 2L version with a more powerful 80W transducer (not available in the UK though), most cleaners on ebay offer 50W:

TATTOO GRIP & ACCESSORIES ULTRASONIC BATH 1.8ltr 50w BRAND NEW CLEANER | eBay

Is it possible to replace the original transducer with a more powerful one if needed, or does it involve changing the electronics as well?

Regards,
dsc.