Why not go for a transfer?
As long as you are up to date with your rent you can choose where you wish to live under the transfer heading.
This is what we did. the 'offer rules' don't apply to transfers. For example if you were on the waiting list for a council property you must take an offer or after, say, the 3rd one or you go back to the bottom of the list, or in some cases are taken off the list for a period of time.
With a transfer you can state what area you wish to live in as long as the HA has properties in that area, or you can, again as long as you are up to date with rent, go for a mutual exchange with some one else in a council or HA property. You can move to any where you wish really
If you have a larger place and want to swap for a smaller one, you'll have no problems in finding either a transfer or mutual exchange.
Your HA may have some incentive scheme where by they offer you a sum of money to move to a smaller property but, no, they cannot evict you unless you do something that warrants them doing so