You get access to the BA Arrivals Lounge in T5 when arriving in T1 longhaul.
I'm arriving in T5 from YVR on BA. However, I wouldn't like to do go thru immigration (I can enter the UK, but I just think that the airside transfer will be easier when travelling in a party of five, three of which have never been to LHR before and don't do well with complicated connection on their own), so I guess I will just go to the LY lounge... Thanks.