Why is London 'the town which Brutus sought by dreams'?

There have been many people named Brutus. The most famous one is probably Marcus Junius Brutus, who is one of the figures discussed by Plutarch and is a character in Shakespeare's Julius Caesar.

Another well-known Brutus is Lucius Junius Brutus, semi-legendary founder of the Roman Republic.

But the Brutus that Wyatt is thinking of is Brutus of Troy, a descendant of Aeneas and legendary founder and first king of Britain. As Wikipedia explains based on Geoffrey of Monmouth's Historia Regum Britanniae ("History of the kings of Britain"),

Brutus then founds a city on the banks of the River Thames, which he calls Troia Nova, or New Troy. The name is in time corrupted to Trinovantum, and the city is later called London.

See Chapter XI in Book One of Historia Regum Britanniae:

After he had nine times repeated this, he walked four times round the altar, poured forth the wine he held upon the hearth of offering, laid him down upon the fell of a hind that he had stretched in front of the altar, and after invoking slumber fell on sleep. For as at that time it was the third hour of the night, wherein are mortals visited by the sweetest sleep. Then it seemed him the goddess [Diana] stood there before him, and spake unto him on this wise:—

'Brute,—past the realms of Gaul, beneath the sunset
Lieth an Island, girt about by ocean,
Guarded by ocean—erst the haunt of giants,
Desert of late, and meet for this thy people.
Seek it! For there is thine abode for ever.
There by thy sons again shall Troy be builded
There of thy blood shall Kings be born, hereafter
Sovran in every land the wide world over.'

In other words, the goddess Diana tells Brute (Brutus) in a dream to found a "new Troy" on an island west of Gaul.