According to THIS WEB PAGE:
"Toilets: At each end of the check-in area, between Zones A / B and E / F and on the left side of the mezzanine level. Disabled facilities, baby changing areas and showers are available." (North Terminal)

Another option you might want to consider is www.executivelounges.com ... the cost is 17.95 GBP ($34) for three hours. The only drawback is that the lounge with showers appears to be in the South Terminal.