If the money is only going to be used for a 'passive' investment such as cash deposit, then the transaction charges of selling the house could outweigh any perceived advantages. Additionally, over the longer term, you are more exposed to inflationary risk with cash.
It might be worth considering what are your longer term goals for your investments. Are you using the houses to fund your retirement? Do you need a wad of cash in the next 5 years? Are you planning to set up your own business? How are your retirement/pension plans? ... etc.
Finally, before deciding to sell (I'm not convinced there is a case for it yet... the case becomes compelling when you move funds from a good place to a better place), are you confident that you have maximised your potential return from the existing properties? The earlier point from Liteweight touches on the below-market rental income. But other ways of maximising your return with a little further equity release could include attic conversion, interior re-design, extension...