they will let you have it where you like! but upto the master socket is there property and their responsibility so when you contact and it has gone wrong they have to come out to fix it.
my bet will be they will rip out your little fix put it right with there own cable etc and then send you a bill for their work at whatever they charge an hour and that probably be expensive.
simple thing have them out to do fttc see what happens but remember the engineers are trained not to talk about costs and money e.g. some people have phoned up for a fault turned out it was a fault on their wiring after the master socket and nothing was said but a £99 bill was sent out by bt.
also hope doesn't stuff up install as they have all necessary gear on board to do it and extra time as well.
but i think you maybe facing extra bill at worse, you may get lucky though and they just put right with no charge but i would be surprised this is bt!