[Nick Robinson]

Well, I can agree with you on one thing and that is it is certainly isn’t a Phylloscopus warbler of any sort. Other than that we are on different planets.
1). The legs are obviously light grey and chunky. Perfect for garden warblers. I’m not sure where you got the perception that garden warblers have dark legs, but that just isn’t true.
2). Garden warblers are smaller and similar in size to willow warbler??? You are joking I hope?!? They are a Sylvia warbler and as such, are the chunkiest of the warblers. Nightingales are quite a bit larger, but they are a sleeker, thrush-like bird, but that is pretty hard to be definitive about in this photograph.
3). Brownish grey – yeah, I’ll give you that, but again you are mistaken in saying that brownish-grey is typical for nightingales. Nightingales always have warm brown upperpart colouration with much cooler white / grey underparts.
4). The substantial representation of a pale eye ring is hard to judge, but it certainly would be emphasised in a ‘face-on’ photograph like this. Nightingales tend to have complete eye rings with emphasis on the upper eye ring. Quite the opposite of what this bird has.
5). I cannot dispute that you may see a hint of a rufous rump, but as you say, ‘Nightingales most obvious feature.’ However, it’s far from obvious and given the warm colouration on the back of this bird the rufous would be pretty obvious.
Unfortunately, you have seized upon a few of the potentially debateable and contentious features that each and all could have a different opinion on and totally ignored the facts that are ‘staring you in the face’ namely, the pale supercilium, the alula and the emarginations on the primaries. Other indicators (other than those discussed above) are the chunky nature of the bill and the warm underpart colouration. In addition, whilst looking again I can also note that there appears to be a greyish collar and this is also a feature of garden warbler and not nightingale.
So, to summarise, whilst the photograph isn’t ideal for identification (and probably makes it impossible to ever be 100% certain of the species of bird in question), if we look at the real evidence and not the subjective details then it’s more likely to be a garden warbler than a nightingale.