Dave Dyson, chief exec of Three UK, said: "UK Broadband gives us an opportunity to expand our ambition to provide high quality and great value internet connectivity for UK consumers".
Its consumer arm, Relish, has 15,000 customers in the United Kingdom - majority in London - which it serves through a mixture of its own 4G mobile spectrum and fibre backhaul.
The mobile firm, which regulators past year blocked from buying rival O2 for £10.2bn, will snap up the business, which operates as Relish and provides broadband for 15,000 customers in the UK. Overall, the company supports 15,000 customers across the UK.
The transaction, which is yet to close, will see UK Broadband Limited become a wholly owned subsidiary of Three UK.
In addition to the purchase price, a deferred £50m will be made available by Three as a credit towards the establishment of a mobile virtual network operator on Three's mobile network.
The mobile operator gets its first, limited slice of the home broadband market as a result of the deal, but CCS Insight Analyst Kester Mann warned it would do little to help its cause in that regard.
Instead, it is arguably the acquisition of spectrum that is more significant.
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Since it is so recent, it stands to reason that it is also the most data-forward: today, its network carries 35% of the UK's mobile data traffic.
Ofcom, in its consultation on the upcoming spectrum auction, said UK Broadband's spectrum share "potentially gives O2 or [Three] another option in terms of increasing their capacity", adding: "This tends to reduce our competition concerns".
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Kester Mann, principal analyst at CCS Insight, pointed out that much of the new spectrum being acquired by Three will not be compatible with most of today's mobile devices, which means it can not be put to immediate use.
It has since expanded to the Reading and Swindon areas, using regional spectrum licences it acquired in 2003.
UK Broadband claims to be "the largest commercial holder of national radio spectrum suitable for 4G mobile services and fixed wireless solutions in the UK".