The Spanish-owned lender, which is the country's fifth-largest bank, said it had booked £60m in credit impairment losses between January and March compared with £13m in the same period last year.
Santander said the UK business, while growing customers, was suffering the effects of the slower economy and weak interest rate environment with fee income falling 2% on the first quarter 2017.
It reported consumer credit growth easing from double-digit rates in 2017 and said corporate loan growth had proved "volatile".
The bank said it would continue to keep an eye on costs over the remainder of the year.
He added: "Cost discipline remains an area of particular focus for management, with targeted actions expected to reduce the cost run rate over the year and deliver operational efficiencies."
The wider Santander group maintained its guidance for the year and reported profit before tax up by 23% to £3.2bn.
It credited stronger results in Brazil, Spain, Mexico and in the US.