Visa waivers probably complicate this, but on the face of it, it depends on what a "reasonable honorarium" is:

'[you must] not receive funding for your work from any United Kingdom source (payments of expenses or reasonable honoraria may be disregarded, as may payments on an exchange basis)'

http://www.ukvisas.gov.uk/en/howtoapply/infs/inf2visitors

Whether this is a good policy is another matter.