We had a similar situation, and our tenancy agreement had a clause which allowed us to sublet. We approached the landlord wanting to get out of the lease early, and pointing out that she would invariably get more for the apartment if she let it immediately instead of waiting for our tenancy to expire. However, she didn't see that and instead said we had to stay and couldn't sublet.
She said she would let it out, but she was asking way above market rental, and only wanted to let it out a month before our lease was due to expire - and 3 months later than we wanted.
According to our legal advice we were entitled to terminate the lease early (we weren't using the diplomatic clause as we were staying in Singapore) and the landlord had a duty at law to mitigate her loss. This means that the landlord must, acting reasonably, try and re-let the apartment at a fair market rate. It also meant that if she re-let the property
she couldn't still charge us rent for the same period.
We were entitled by our lease to sublet the property and the landlord could not unreasonably withhold permission.
As we had the law on our side though, we advertised the property for a short-term let, and had the landlord meet the prospective tenants. We sub-let the property for the remainder of our lease, and we continued to pay the rent, and received the (same) rent from our tenants.
We were also concerned that our landlord may keep our two months deposit (I have heard of this happening on a regular basis and for very flimsy excuses). We decided not to pay the last month's rent (trying to reduce how much the landlord might want to keep as she had been a quite difficult landlord).
She actually didn't say anything about that, and we also received back half of our security deposit (she kept the other half being the month's rent we owed her), and she was pleased with the condition of the property when we returned it to her.
The only long-term problem I have had is I still get real estate agents ringing and sending me SMS message about apartments for sale or rent, and wanting to know whether I still had the apartment (from mid 2008) and that is very frustrating. I have removed all adverts that I placed but obviously once they get your number ...
The people I sublet to didn't come through an agent, I can't remember where now though.
I used this forum to advertise, and some real estate forums as well.