Electronic voting machines would make counting the vote quicker. It may also may it more user-friendly for voters where there are large numbers of options on the ballot, or where ranked voting (such as STV) is used rather than first-past-the-post. An electronic voting machine could take voters through the process of ranking candidates step by step rather than relying on them to be familiar with the system.
Against that, as anyone who has found themselves in a queue for the self-service checkouts at a supermarket can testify, there is a large number of people who seem unable to read and follow instructions on a screen. If electronic voting were to replace traditional ballot papers, polling stations would probably need more staff to be on hand to assist voters unfamiliar with the technology. There may also be privacy implications if someone was unable to use a machine without a staff member present to guide them.
The other main disadvantage would be the lack of a publicly visible audit trail. Allegations of faulty voting machines in the USA affecting the outcome are not uncommon. For that reason, any electronic voting system used in the UK must not rely on closed, proprietary software. Both the mechanisms and source code must be open and freely available for security researchers to inspect.