I presume their transmitters and receivers are co-located. I would think that their transmitters are a lot more powerful than an aircraft's radio. Which might explain why you can hear them, but they can't hear you.
As for hearing aircraft on the other side of the country.....this is because when London info are operating with one controller on all three frequencies, their system re-transmits calls on the other frequencies.....ie if someone calls on 124.75, this call gets transmitted by their transmitters on 124.6 so everyone there knows not to step on the transmission. ....and vice versa, so that is why you can hear aircraft hundreds of miles away.
Your radio must be ok if you got comms with Southend over the sea.