She is an earning adult now and needs to learn it is the right thing to do to contribute financially and physically to family life, pay for her own things and also budget/not live above her means.
Switch the phone payment to her now.
Tell her to set up a weekly or monthly DD to pay her digs money (depending on when she gets paid)
You are doing her no favours letting her get used to the level of disposable incoming she currently has with no commitments.
Tell her she is expected to help with housework, whether that is clean the bathrooms, keeping her room clean, helping with supermarket shop, vacuuming, making dinner once/twice a week, laundry etc.
Ideally she should have been expecting this as you would have been working on instilling these values into her over the years, ds(14) already knows once you start working you pay your own way (even if it is only a token amount) and has his age appropriate chores (general keeping tidy, loading/emptying dishwasher, his own room, vacuuming, stripping bedding and putting on to wash, sometimes windows etc).
Has it come as a shock to her so might take a while to sink in and need a bit more discussion or is it just because its actually happening now? (genuinely interested so I'm prepared for same from ds!)
If ds disagrees in the future I might mention rough mortgage is ££, utilities is ££, council tax ££ food ££ etc but there is no way I'd be going through household income/outgoings to justify taking £20 a week, it is only a token payment and does not in any way pay for her costs and she should respect that. Otherwise I'm afraid I'd tell her the same my dad told me (and I as a teenager I obviously hated at the time), if you can find somewhere cheaper then you are free to go.