Alienware upgrade RAM issues

Firstly hi everyone I'm new to the forums and not all that tech savy when it comes to computers so please go easy on me ha-ha

I've been looking to upgrade the ram in my computer so used Mr Memory and Crucial to point me in the right direction for compatible RAM and both said I needed DDR3 128000, 1600mhz 204 pin so I had a shop around and found some a little cheaper anyway the issue is I get two beeps which means no ram detected, I done some more research and have now found the new 17 only takes DDR3L ram which is a lower voltage and there's no backwards compatibility I've read. So besides having to send this RAM back where do I find DDR3L ram as I can't seem to find a stockist in the UK?

Thanks for any help