Welcome to the forum. Although you can post in any forum, the USA forum is here in case of local regs or laws
power flow will clean very oxidized copper with no problems at all, id trust it over laco if you can get the fitting on.
Reply to the thread, titled "How do you apply your flux?" which is posted in UK Plumbers Forums on Plumbers Forums.
We recommend City Plumbing Supplies, BES, and Plumbing Superstore for all plumbing supplies.