Not sure that a lack of signature makes a difference; if you can ring up a phone number that takes the place of returning the form (you can't sign a phone call) the update occurs.
Without a signature to the contract there is only one consenting party to the contract, ie the state, which would make it void. (one sided contracts in english law are considered unenforceable, in that while the contract may have been valid at the time of agreement, the courts will not require you to comply with the conditions of the contract)
The signature provides them with another consenting party to the contract, ie you, and provides them with someone who can be prosecuted in the information is found to be false. (remember, contracts have to be between two *consenting* parties according to english law, no signature = no agreement which in turn equals no consent)
Of course this applies only to the forms posted to you, not online or by phone as I am unaware of exactly how those systems work or even if it possible to say you wish your details are removed from the electoral roll when using those systems. It is quite clearly possible however using the good old fashioned pen & paper system which has survived for many centuries.