I was looking at website of British Summer Time Hyde Park event sponsored by Barclays and found this. The application is intended for this event ending on 13 July but actually the wristband will work further beyond wherever you see the contactless symbol.
From what I understand this wristband is like prepaid contactless payment gadget where you'll need to top up before you can use it to pay for transactions. Basically like contactless card. I copy paste T&Cs below. Also you can get offers on stuff you pay using the band such as 10% off at Hotel Chocolate and 15% off at Ryman & Robert Dyas. Full offers here https://bpayband.co.uk/Offers.aspx.
And its not illegal to wear it without credit in it.
The direct link on the title brings you to the application form for BLACK bands which affiliate with BST Hyde Park event. But again, the website clearly says that you can use it beyond the event. For PINK bands affiliating with London Pride event please go here https://www.bpayband.co.uk/Pride/RegistrationPayBand.aspx.
Make sure you open either the black or pink form at one time. Don't open both at the same time.
I just applied for one myself today so I cant give testimony.
Summary terms and conditions
1.1. Your bPay band can be used to make contactless transactions up to £20 where you see the contactless symbol. It cannot be used for gambling transactions.
1.2. Your bPay band will expire on 30 June 2015. Any unspent money will be returned to you after this date unless you ask us for it earlier.
1.3. You can only spend money which you have already added to your Account.
1.4. You will need to use a Visa or MasterCard® credit or debit card to add money to your bPay band Account.
1.5. The minimum initial amount you can add to your Account is £25. The minimum amount that you can add thereafter is £5. The maximum amount you can add at any time is £100. The maximum balance you can have at any time is £200. You can reload your account (including by auto top up) up to 4 times, with up to £400, a day.
1.6. You can set up an auto top up facility, so that if your balance drops below £20, we will automatically add an amount of up to £100 from your card to your Account, subject to the daily maximums above.
1.7. You should read the How We Use Your Information section of the terms and conditions. By accepting them you are agreeing to us processing your personal data in the ways we set out. We will use your personal data to manage your account, for statistical analysis and behaviour scoring, to develop our services to you and others, and marketing if you consented. We will keep your personal data confidential but may share it with the Barclays Group, people providing services to you or us, or with people we are providing services in conjunction with, always ensuring they keep it confidential too.
1.8. If you change your mind you can cancel this agreement in the 14 days after you register by calling us on 0333 202 2065. We will return any unspent money on your account.
If you get error messages when applying, see comment 110, below.
Added 15th July -- the discount offers mentioned above have now expired, but this one is still active: "10% off total bill Bright Courtyard Club from Sat 28 June to Sat 28 July (excludes set menus) at Baker Street, London."