It's ALL about the spectrum
The customer base of 15,000 is worth between £2m and £3m. You might attribute some value to the Relish brand but not much. The value is in the spectrum.
The magic thing about 3.4GHz is that it's flavour of the month for 5G and has been allocated some experimental status. It's not enough bandwidth for Ericsson's vision of 5G which wants 500MHz of contiguous spectrum and so is looking at much higher frequencies but it's plumb where Huawei see 5G.
Unfortunately this move goes to re-enforce Ofcom's (unstated) reason for blocking the Three/O2 sale. If the two companies had merged there would have been one fewer customer for the (delayed for over a year), auction (https://www.theregister.co.uk/2015/10/26/ofcom_spectrum_auction/) for 3.4GHz and 2.3GHz. Ofcom is all about "recognising market value" for the spectrum, and Three has just put a price on it. And Ofcom has to be disappointed at how low that is.
Ofcom claims it's about keeping competition in the market but if you look at what has happened at retail, all the real networks have increased prices. Only the MVNOs keep the MNOs under price pressure.
The 2.3GHz is now interesting because there are quite a lot of phones which support it.