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.
(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.