Ahead of his visit to the UK and France on Wednesday, Prime Minister Prayut Chan-o-cha on Tuesday denied wanting to discuss bringing fugitive former premiers Thaksin and Yingluck Shinawatra back to face justice in Thailand.

The premier said the focus of his trip to will be the economy, trade and investment.

The trip may now see the junta chief face awkward questions over the use of the death penalty as well as Thailand's wider human rights record since he seized power in a 2014 coup.

Gen Prayut, however, defended capital punishment on Tuesday, telling reporters that in order to maintain peace and order in society, executions are "still a necessity and what people want."

He has not addressed the possibility that the issue will be raised by his hosts in Europe.

"There are many serious cases happening today. This visit follows the resolution of the Foreign Affairs Council of the European Union that wants to upgrade relations with Thailand although this is not 100% yet," he said.

The EU's Foreign Affairs Council on Dec 11, 2017 decided to resume political contacts at all levels with Thailand in order to facilitate meaningful dialogue on issues of mutual importance, including human rights and fundamental freedoms, and the road towards democracy.

While some dissidents are planning to rally against him during his trip, Gen Prayut said this is their right and all countries have laws to take care of official guests.

"Please allow me to travel happily because I have to ensure understanding with foreign countries on many issues especially the economy and the road show for the Eastern Economic Corridor," the prime minister said.

In the early morning in the UK Wednesday, Gen Prayut will meet with executives from Thai companies operating there. The premier is scheduled to meet with British PM Theresa May at her residence and office at 10 Downing Street in the afternoon.

A free-trade agreement (FTA) is on the agenda to boost trade and investment.

KonthaiUk Facebook page said that Giles Ji Ungpakorn, an academic living in self-imposed exile, will lead a protest at Downing Street at 2pm local time on Wednesday and another protest tomorrow at a hotel where Gen Prayut plans to launch a seminar.

Deputy government spokesman Werachon Sukondhapatipak said Gen Prayut is also scheduled to open a seminar, "Transforming Thailand," in London on Thursday. Gen Prayut will then visit Pearson Plc, a British multinational publishing and education company.

The prime minister is set to meet Airbus Commercial Aircraft president Guillaume Faury in Toulouse on Friday, where Airbus Commercial Aircraft and Thai Airways International Plc will sign a deal on a joint venture on maintenance, repair and overhaul.

Gen Prayut plans to meet French President Emmanuel Macron on Monday and preside over the signing of an agreement between Thai Airways International and Airbus Commercial Aircraft to open an aircraft maintenance and repair hub at the civil-military U-Tapao airport in Thailand, according to the Prime Minister's Office.

Gen Prayut will also take part in a business conference organised by the Board of Investment and the Mouvement des Entreprises de France on Monday.