I think this is a very complex issue on which it is very easy (and natural) to make simplistic value judgements depending on your own lifestyle and observation point. I work in a field that's somewhat aligned to this, but don't profess to have a PhD or anything in it, but these are my thoughts:
1) First of all, I think that 'poverty' must be seen as how it manifests now, in our society. I don't think it's helpful to compare poverty as it is in 2018 to either past eras or to other countries, as when it boils down to it I see poverty as a lack of financial means to meet one's needs as defined by nature and to fully participate in one's society.
2) Yes, the <60% of national average income is a simplistic measure, but its value is that it is easily measurable, and gives us the chance to compare amongst different areas using the same metric and (hopefully) put measures in place to help people when they're at 39% , before they get to 5%
3) One of the greatest threats to wellbeing is social isolation, which can be a vicious spiral. Given that we no longer live with the extended support structures that we once did (for the most part), I have no problem with people in poverty having a smartphone. Not least because the cost of my 4GB SIM only contract comes in at £11 per month less than line rental alone for a landline, if that's the alternative we're looking at. And that's not even counting the value of a smartphone for applying for jobs etc.
4) It really doesn't help how much poverty tax, seen and unseen, there is in this country, and how much predation there is - from gas / electricity meters, to the shops that are at both ends of my high street charging hideous amounts of interest so that people with low incomes / poor credit can get a bloody sofa to sit on for £15 / week for the next 50 months (that may be hyperbole).
As an anecdote - I've lived in industrial northern cities for the vast majority of my life (currently in denial about what that's doing to my lungs). A friend of mine who grew up in St Albans moved to Manchester for his PhD, and I got a text from him after a week asking why there were so many more frozen food shops in the North. It blew his mind that there are towns that are still suffering so much from the closure of factories / mines, and took longer to get his head around the fact that you might want to pay £1 for a guaranteed kilo of cauliflower that will last for months in the freezer, should something get cocked up with your benefits / not get any hours in your job this week, vs £1 for a fresh cauliflower of which most is leaves.