Nick Clegg has said the Liberal Democrats should stop apologising for their time in coalition with the Conservatives, in a sideswipe at the call from the future leadership favourite, Jo Swinson, for the party to “own the failures”.

Speaking at the Lib Dems’ annual conference in Brighton, the former deputy prime minister said: “One of the golden rules of politics is don’t say stuff that reinforces your opponent.

“We went into coalition eight years ago, there is something quite Lib Dem about still thinking about the things we did or didn’t feel comfortable with when we entered into government so long ago.”

Clegg said he did not believe mea culpas to be election-winning strategies. “In the end, you have to give a clear message; are you basically proud of who you are, or not?” he said.

“There is this narrative hung round the Lib Dem party’s neck, which is just false, that we sold our soul, merrily went along with a savage, ideological approach to austerity which deliberately penalised the poor. It is simply not true.”

Swinson, the party’s deputy leader, who was a business minister in the coalition, cited the bedroom tax and hostile immigration environment as regrettable policies. “When they fought dirty, we were too nice,” she said.

The main draw on the conference floor on Monday was the anti-Brexit campaigner Gina Miller, tipped as a potential future leader of the party if changes allowing non-MPs to stand are voted through.

However, the businesswoman, who led the legal fight for a parliamentary vote on article 50, said she would not stand for leader or join the party.

“Today I speak to you as a friend and someone who feels a bond with you on many issues, but may I say straight away, particularly for the benefit of the journalists here in this hall who have been doing a great deal of speculating lately, I am not addressing you as your leader-in-waiting,” she said.

“Truth be told, I am not a member of your party, or indeed any party now, but I want, all the same, to see the Liberal Democrats thrive, because in a healthy democracy we need a strong third party, every bit as much as we need a strong opposition, and, for that matter, a strong government and a strong prime minister.”

Speaking to journalists afterwards, Miller expressed irritation that she is often portrayed as wanting to “stop Brexit”. She said: “I wish you would all stop saying this, you know, it’s really not helpful: stop saying ‘stop Brexit’.”

Miller also expressed doubts about the phrase “people’s vote” as a slogan for those campaigning, as she is, for the public to be given a say on the final Brexit deal.

“I don’t like the phrase,” she said. “It’s not definite enough. It doesn’t have enough detail behind it.”

On the conference floor in Brighton, Lib Dems backed a move to unilaterally revoke article 50 if the prime minister cannot secure an exit deal by March, to avoid a no-deal scenario.

Party members also supported an activist’s amendment by which the UK could opt to stay in the EU without a second referendum or general election, though with some tacit backing from the leadership.

The policy seeks to extend the article 50 period in an attempt to secure another referendum. However, the policy states that if no deal has been reached and no referendum is in sight, the UK should “withdraw the article 50 notification” if the EU will not allow an extension.