The Transitional Element is for those who already have SDP before they claim UC.

You cannot get the SDP while you don't have anywhere to live on your own.
When you do find somewhere to live on your own you will have to claim UC to pay the rent, so cannot get the SDP then.

if I move in a place claim sdp for 2 weeks then claim uc is this allowed?
What you ask is technically possible, if done correctly - but how are you going to show that you can afford the rent before you claim UC?

You may be able to afford a fortnights or even a months rent without claiming UC - But no one is going to give you a place to live if you can't show that you can pay the rent on an ongoing basis.

In your situation you are most likely to be moving to Social or even Supported Housing.
You will need to claim UC to cover the rent before/at the same time that you can get a Social/Supported Housing let.
They have rules and will not let to someone who cannot show that they will be able to afford the rent in future.

(I've just moved into a Social Housing flat and they wanted to see, and double check, that my benefits would cover the rent before I was even offered the tenancy).

So while what you ask is technically possible, in your current homeless situation practical considerations make it almost certainly unworkable for you.
(The only vanishingly small option is if you can get the housing providers welfare officer on your side and supporting what you would want to do).

Again - In your situation you should forget all about SDP (and UC Transitional Element).
Concentrate your efforts on getting into housing.

Please answer the question about where you are currently living?
The answer to that will let us know what you should do next for the best, but I expect that will be to claim UC now.

You have been told at least twice that you have already lost money by not claiming UC earlier, that's now a lot of money that you could have been getting but have lost because you didn't claim UC when advised to.
The longer you leave it the more you are losing.