Registering as Self-Employed but how long have i got?

My first physical trade will be on Monday, 3rd December. It is on that date that i shall be informing the HMRC that i have gone self-employed.
I left my previous job in mid July and want to know whether i can back claim my 拢5/month rates for using the house as an office and a portion of the gas/electricity for rent & rates. However, the question is whether i can back claim these to mid July or can i only start claiming them from the date that i have gone Self Employed. I could state that i went self employed 鈥榓lmost鈥?3 months ago (& not incur the late penalties) & if i did that would i then be able to back claim for those 3 months. I suppose, in a nutshell, the question is: Although i wont be trading until 3rd December i have been setting up the company and establishing business contacts since mid July, would i therefore be able to back claim rent & rates during this period of 鈥榗ompany formation鈥?or would it have to be from the date that i have gone 鈥楽elf Employed鈥?on paper with the HMRC?

"One Step Down From God" is 100% correct in his answer.
Good luck with your new business venture.

You had 3 months from the date you went self employed - regardless of the fact that you have just made your 1st trade. Reading what you have written you went self employed in mid July, so you had to tell them no later than mid October.

Whether you have commenced trading or not is a question of fact.
Preliminary activities in setting up a business do NOT count as trading.
You commenced trading on 3rd December. Any expenditure in the months before then is allowable as a deduction, subject to certain restrictions. See ICTA 1988 s 401. (Pre-Trading Expenditure).
You have 3 months from the end of the month of commencement to notify HMRC that you are in business and liable to Class2 NICs.