Getting a Conventional Mortgage for a Short Period of Time

Assalamu Alaikoum Wa Rahmatou Allah Ta3ala wa baraketouhou. Bismi allahi arrahmani arrahim. Dear Scholars can you please clarify and if possible give me a definite answer to my Question which is in regards to Mortgages. I live in the UK I have just married, the house we live in is a 1 bedroom house and we pay £600 rent a month. I have approached islamic banks for Mortgage (Amamnah finance), as I do have about 15% of the value of the house. even then they said I wont be able to afford it based on my salary, and it is true I will not be able to afford it at this time. I approached conventional Banks for conventional Mortgages and they offered us a good mortgage with repayments that we can actually afford and it will give us the chance to buy a bigger house than the one we live in. in regards to darurah of the matter, My wife is finishing her studies at University and she cannot actually do her work properly because of the lack of space, we are happily maried but the space issue gets to us and sometimes gets in the way of our marriage. I have asked about this and I was told that I do not have a darurah in this case because I have a roof to live under. My question is as follow, My intetion is to get a convetional mortgage for a short period of time to get the space and get a house which we can actually call home, inchallah and with allah's help improving our financial situation we will move to an islamic mortgage when we can afford it, I am working towards getting a better higher paid job? Second is my situation a darurah or is it not? Jazakoum allaho Khier