HD, En route ground ATC stations in the UK transmit considerably more than 5W ERP, I presume the 5W is from the Aircraft but have no knowledge of radios in aircraft.
For increased coverage of a specific ATC frequency a system of Transmitter Offsets are used. i.e for a 3-leg channel one ground station would use the exact frequency while the other two would be + or - 7.5KHz the centre frequency. Similar techniques are used for 2 and 4 leg channels.
Using this system increases the area coverage of a specific frequency.
Regards P