What is the third option beside 'accept' and 'reject' on a ballot?

"Abstain," as you suggest, is widely-understood, at least in the UK.

See this Sky News article on the EU withdrawal bill which includes:

However, there were still rebels among Labour and Tory MPs, along with some notable abstentions.

You specified MPs, but then asked for "any English spoken country". In the US Congress (and generally in the USA), legislative votes are typically recorded as "Yea" (yes/for passage), "Nay" (no/against passage) or "Present".

For example, this link is for the most recent set of votes in Congress of type Yea and Nay.

The term Abstention sounds best here, as you suggested abstain; it denotes:

an instance of declining to vote for or against a proposal or motion.

It's not political representatives as you asked, but a similar example that may be interesting is the procedure used (in US) for voting by stockholders of a publicly-owned company, usually at an annual meeting, but occasionally at a special meeting called to consider a merger or similar.

Shares can be voted explicitly yes/for, no/against, or abstain/withhold at these events. However, many shares are "held in street name", meaning they are registered as "owned" by the stockbroker who bought them on behalf a client who is the true "beneficial" owner; this allows purchases and sales to be done electronically which is quick and cheap rather than having to carry or mail paper certificates as was formerly done. (I'm old enough to remember my father in the 1970s doing that.) Since the broker is only a custodian of these shares, it must vote them as instructed by the client -- and if the client doesn't give instructions, as they often don't, the broker is permitted by regulations to vote on matters classified as "routine", which are now few, but not anything else. Shares not voted for this reason are in a fourth status called "broker non-votes" and depending on the company bylaws and corporation law for its state these non-votes sometimes count the same as abstentions but sometimes are treated differently.