STOKE-ON-TRENT, ENGLAND - FEBRUARY 20: British Prime Minister Theresa May is escorted by Emma Bridgewater and Stoke Central by-election candidate Jack Brereton, as she arrives at the Emma Bridgewater pottery factory in Hanley on February 20, 2017 in Stoke-On-Trent, England. The prime ministers visit comes ahead of crucial by-elections in Stoke-On-Trent and Copeland this week. (Photo by Christopher Furlong -Pool/Getty Images)

STOKE-ON-TRENT, ENGLAND - FEBRUARY 20:  British Prime Minister Theresa May is escorted by Emma Bridgewater and Stoke Central by-election candidate Jack Brereton, as she arrives at the Emma Bridgewater pottery factory in Hanley on February 20, 2017 in Stoke-On-Trent, England. The prime ministers visit comes ahead of crucial by-elections in Stoke-On-Trent and Copeland this week.  (Photo by Christopher Furlong -Pool/Getty Images)

STOKE-ON-TRENT, ENGLAND - FEBRUARY 20: British Prime Minister Theresa May (C) is greeted by Emma Bridgewater and Stoke Central by-election candidate Jack Brereton, as she arrives at the Emma Bridgewater pottery factory in Hanley on February 20, 2017 in Stoke-On-Trent, England. The prime ministers visit comes ahead of crucial by-elections in Stoke-On-Trent and Copeland this week. (Photo by Christopher Furlong- Pool/Getty Images)

STOKE-ON-TRENT, ENGLAND - FEBRUARY 20:  British Prime Minister Theresa May (C) is greeted by Emma Bridgewater and Stoke Central by-election candidate Jack Brereton, as she arrives at the Emma Bridgewater pottery factory in Hanley on February 20, 2017 in Stoke-On-Trent, England. The prime ministers visit comes ahead of crucial by-elections in Stoke-On-Trent and Copeland this week.  (Photo by Christopher Furlong- Pool/Getty Images)

STOKE-ON-TRENT, ENGLAND - FEBRUARY 20: British Prime Minister Theresa May (C) and Stoke Central by-election candidate Jack Brereton, is escorted by Emma Bridgewater (R) during a tour of the Emma Bridgewater pottery factory in Hanley on February 20, 2017 in Stoke-On-Trent, England. The prime ministers visit comes ahead of crucial by-elections in Stoke-On-Trent and Copeland this week. (Photo by Christopher Furlong- Pool/Getty Images)

STOKE-ON-TRENT, ENGLAND - FEBRUARY 20:  British Prime Minister Theresa May (C) and Stoke Central by-election candidate Jack Brereton, is escorted by Emma Bridgewater (R) during a tour of the Emma Bridgewater pottery factory in Hanley on February 20, 2017 in Stoke-On-Trent, England. The prime ministers visit comes ahead of crucial by-elections in Stoke-On-Trent and Copeland this week.  (Photo by Christopher Furlong- Pool/Getty Images)

British Prime Minister Theresa May (R) and Stoke Central by-election candidate Jack Brereton (L) visit the Emma Bridgewater pottery factory in Hanley in Stoke-on-Trent, central England on February 20, 2017. The prime minister's visit comes ahead of the by-election for the seat of Stoke-on-Trent Central which will take place on February 23, 2017, / AFP / POOL / Christopher Furlong (Photo credit should read CHRISTOPHER FURLONG/AFP/Getty Images)

British Prime Minister Theresa May (R) and Stoke Central by-election candidate Jack Brereton (L) visit the Emma Bridgewater pottery factory in Hanley in Stoke-on-Trent, central England on February 20, 2017. 
The prime minister's visit comes ahead of the by-election for the seat of Stoke-on-Trent Central which will take place on February 23, 2017, / AFP / POOL / Christopher Furlong        (Photo credit should read CHRISTOPHER FURLONG/AFP/Getty Images)

STOKE-ON-TRENT, ENGLAND - FEBRUARY 20: Pottery worker Lisa Cooke demonstrates sponge decoration to British Prime Minister Theresa May and Stoke Central by-election candidate Jack Brereton, as they escorted by Emma Bridgewater (R) during a tour of the Emma Bridgewater pottery factory in Hanley on February 20, 2017 in Stoke-On-Trent, England. The prime ministers visit comes ahead of crucial by-elections in Stoke-On-Trent and Copeland this week. (Photo by Christopher Furlong -Pool/Getty Images)

STOKE-ON-TRENT, ENGLAND - FEBRUARY 20:  Pottery worker Lisa Cooke demonstrates sponge decoration to British Prime Minister Theresa May and Stoke Central by-election candidate Jack Brereton, as they escorted by  Emma Bridgewater (R) during a tour of the Emma Bridgewater pottery factory in Hanley on February 20, 2017 in Stoke-On-Trent, England. The prime ministers visit comes ahead of crucial by-elections in Stoke-On-Trent and Copeland this week.  (Photo by Christopher Furlong -Pool/Getty Images)