The Kingdom of Heaven actor, 28, has enjoyed an on/off relationship with the 22-year-old beauty since January 2003, after they met filming an advertisement for US clothing giant Gap.
An onlooker tells British newspaper The Sun, "They were picking out engagement rings and she was trying them on. They popped into lots of flash jewellers on New Bond Street. But they were in Bulgari and Van Cleef + Arpels for the longest period.
"They had huge smiles on their faces and were kissing and cuddling."
In September (05) Bloom denied engagement rumours but hinted he would soon be ready for marriage, saying: "None of that stuff is ever true. But that doesn't mean I don't love someone and that I'm not going to get engaged one day."