This is incorrect. The US-UK bilateral air services agreement (commonly referred to as Bermuda II) allows only two designations from each of the US and UK to use LHR (and in fact also limits the US destinations that can serve LHR). Between the UK and other countries, usage of LHR depends on the agreement between the UK and that country. The EU open skies agreement means that an unlimited number of carriers from any EU country could use LHR, with the caveat being that they would need slots. Slot control is indepenent of the bilaterals.