We are going to make the assumption here that the money being saved is for a future purchase. Your concern of having the money protected at all costs is understandable, as you want to make sure you are able to purchase a home when you finally find the right one.
Other than putting the money in your mattress, there are a few options:
The current guarantee for deposits is £50,000, £100,000 if it is a joint account. Assuming that several hundred pounds is at stake, this would require the opening of several accounts if you wanted to make sure the funds were guaranteed.
For your trouble, you would earn less than one percent interest on your money.
Another alternative is to open a National Savings and Investment account. There is no limit on how much you can put into this account as they guarantee that 100 percent of all funds are secure. The interest rate on these accounts is still rather low, but at two percent, it is a marked increase over your other option.
Keep in mind here that it is unlikely that banking system is going to completely collapse any time in the near or far future. Times might be tough, but this is something that our government will not allow to happen.
It may be wise to look into some other options that will still allow quick access to your cash, but allow for a much higher interest rate.