Just reroute the Thames temporarily, cover that **** in mega-thicc steel walls and just let it blow up. Easy peasy.
construction of said walls risks setting it off. The tidal wave that the blast would create would jump the thames barrier and flood large swathes of the estuary and up into central london. They're not going to go near it with anything making vibrations. The hope is the seawater is degrading the material enough to render it ineffective. The main concern is the corrosion of the hull that is nearing the point where it could split and allow shells to spill out and clatter against each other. They are not happy about the prospect of all that movement. It's too volatile to get in there and may cause the event you're trying to prevent by doing so. Ergo, waiting game and hope for the best. Not ideal, but all they can do right now really.