The FTSE 100 edged up by just 1.88 points to 7,292.37.
On a day nicknamed "Super Thursday" because so many retail updates were released, the biggest faller was Primark owner Associated British Food, with shares down 4.52%.
Dixons Carphone, Next, Shire and Carnival were the other big fallers.
Total sales at Primark rose 11%, with like-for-like sales "good". However, it said like-for-like sales were down in Germany and the Netherlands.
Tesco's shares fell 1.3%, despite the UK's largest supermarket reporting a 0.7% rise in UK like-for-like sales over Christmas.
Investors welcomed news of better-than-expected sales at Marks and Spencer in its clothing division, sending the High Street giant's shares up 1.3%.
Underlying sales in the clothing and homeware division rose 2.3% - well above analysts' expectations for about 0.5%.
In the FTSE 250, shares in JD Sports jumped 7.5% after it reported a strong increase in sales and raised its full-year profit forecast.
Also on the FTSE 250 Debenhams rose by 5.4%, while Dunelm lost 6.9%.
On the currency markets, sterling rose against the dollar earlier on Thursday as the US currency weakened following a news conference from US President-elect Donald Trump on Thursday.
The dollar fell after Mr Trump failed to give any further detail on his economic plans.
However, it subsequently fell back on the news that Prime Minister Theresa May will give a major speech about her Brexit plans next Tuesday.
The pound was down 0.04% against the dollar at $1.2206, and down 0.81% against the euro to â‚¬1.1447.

So called "tandem burials" happened at a limited number of hospitals in the Republic of Ireland up to the 1980s.
It is understood this happened in what the authorities called "exceptional circumstances".
This might be if a newborn baby died and options for burial in a hospital or religious plot were not selected.
The remains would then be placed in the coffin of an adult who had died.
Details of such burials were set out in documents obtained by RTÉ News under the Freedom of Information Act.
In a letter to the former secretary general of the Department of Health, Dr Ambrose McLoughlin in August, Irish Health Service Executive director general Tony O'Brien  said it was not certain that the families of both of the deceased would always have been informed of the practice.
In the letter, Mr O'Brien said: "Our understanding is that the remains would have been placed with adult remains and that, ideally, the awareness and understanding of both families involved would have been sought, though this is not guaranteed."
It is understood that the practice may have occurred in a number of cases to ensure that a baby was buried in consecrated ground.
At a time in Ireland, the Catholic Church's tradition stipulated that if a baby was born and died before baptism, they could not be buried on sacred ground in a cemetery.
The souls of such babies would go to Limbo - an in-between place where they could neither see God nor be reunited with their parents.
In 2007, Pope Benedict XVI revised the concept of Limbo.
(Close): London's main share index has closed at a record level for the 11th day in a row - a feat it has never managed before.
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In Ireland, dead babies have been buried in the coffins of adults to whom they were not related, according to Irish broadcaster, RTÉ.