Abhisit's invitation to speak to Parliament debunked: official UK Parliament response.

Brief post but just had a response from the UK Parliament's press office. And it is unequivocal. Abhisit was not invited by Parliament nor was he invited to speak to Parliament nor was the person who invited him, chair of the Thailand All Party Parliamentary Group (APPG), Roger Godsiff MP, even an official representative of Parliament. That he and Korn claimed they were invited by Parliament is quite risible. They were invited to a simple reception inside the Parliament building hosted by an MP. 

With regards to your query, foreign politicians often address parliamentarians at meetings of APPGs or receptions, but this should not be taken as an invitation to address Parliament as a whole, as APPGs have no official status within Parliament.  An Address to Parliament normally takes place as part of a state visit, and involves a speech to both the House of Commons and the House of Lords.  A formal address is a comparatively rare event, and you can see a listing of some of the most recent ones here:http://www.parliament.uk/documents/commons/lib/research/briefings/snpc-04092.pdf.