You can claim an amount based on the number of rooms used - only counting reception and bedrooms - for Heat & light, Council tax, Insurances, Telephone.
I have been quoted £8 per month which i more or less equates to £2 per week however you have to be careful as it will have capital gains tax implications if you declare you use a number of rooms for business purposes. As to backdating, i'm not sure however i'm sure someone on the forum will be able to tell you.
Only expenses for the current tax year (or basis period) can be included on the current tax return. Claiming 52 weeks worth on the tax return is fine, but if you want to back date a claim its a bit trickier. Assuming you'd submitted a return before I believe you can amend the relevant section and resubmit it to HMRC if its within 12 months of the original submission.
If its for more than 12 months then you will need to write to HMRC and ask to claim error or mistake relief. Normally you can do this up to five years. If you'd want to do this is another matter. It may raise more questions with HMRC than is worth the hassle for relief on £2 per week.
