www.iva.co.uk

If you were lodging but then became romantically involved, would you have to declare that to your IP as cohabiting or lodging? Is there a vast difference in what your allowed expenditure would be?

Any change in circumstances need to be reported to your IP.
However to answer your question re the difference; your partners income will be used to ensure fair split of household bills etc based on income and my understanding this is similar to a "lodger" so no not alot of difference

Just make sure the lodger pays rent

It is cleaner and easier to ring fence a lodger's expenses than a co-habitee, but at the end of the day the financial result is about the same.