While Orange and T-Mobile did the dirty and got hitched, Vodafone and O2 are looking lovingly into either other's mast-shaped arms. OK, it's nothing as extreme as EE's love-in, but the OFT's approval should see both O2 and Vodafone get a signal boost, for both 3G and any new 4G networks.
The two networks are essentially signing an infrastructure sharing deal, which lets them partner up on mast sites. That means, in theory, more mast sites for each network, so coverage should be better. It should also bestow the ability to roll out a solid 4G network faster for both networks, sharing LTE equipment, which is great news. Both networks will still exist as disparate entities though, with their own spectrum and networks -- there won't be any hoping between O2 and Vodafone like you could on the old EE networks.
Image credit: Mobile mast from Shutterstock