It really depends what your strategy is as to who you'd favour. EE has more spectrum than O2, but it's all up in the gigahertz where it can be great for providing fast data in well built-up areas but a real struggle to provide coverage in more rural areas.
O2 has less total spectrum but quite a chunk down at 900 MHz where rural coverage is much easier, but providing bandwidth a bit more challenging. It also has a much better established network of sites, as previously mentioned. Monetary value of their spectrum may well be greater.
Only Vodafone is as well positioned spectrum wise, but hasn't really capitalised on it. Since the Cellnet days, the O2 network has offered the best countrywide coverage and I'm guessing BT would target universal coverage rather than blisteringly fast hot-spots of coverage in the cities and shite coverage elsewhere.
Then again, BT already has infrastructure reaching to every telegraph pole, and a lot of that is now fibre, so maybe they would figure that they are best positioned to capitalise on the higher bands by plastering the place with small cell sites. After all, a big cell site at every exchange is a bit 1990's. When consumers expect tens of megabits, the cells have to be smaller.