Not sure if you linked to something more specific (an actual contract deal, showing omonthly fee and so on - when I clicked the link it ground to a halt because of 'redirectingat.com' website)
Anyway, mobiles.co.uk and some other names (eg e2save.com) are mail-order firms owned by Carphone Warehouse, just so you know... offering slightly different deals or colours or whatever.
Not sure why your son using Orange would have any effect, unless you want to also use Orange for him to have you as a 'special friend' on his account (sorry, out of touch with 95% of promotions).
If you are considering move of network (and presumably taking your phone number with you), then you might find a Vodafone deal on e2save.com
Vodafone 24m contract. No fee for the phone, pay 30 pounds a month (but get refund of 28.01 seven times during the contract, taking cost down from 720 pounds to 523.93 ... the equivalent of paying 21.83 a month).
They offer 1 GB of data, 'unlimited' texts and 300 minutes of calls.
I know it means sending off past statements/receipts to get the cashback deal, but if you want to be able to access the internet when away from home or office, this contract has data included, and it would be an extra cost.
Why Vodafone ? Only that the latest version of Android (v2.2) is due to be on Vodafone phones quite soon, so one to look out for (unless you spot a better deal with v2.2 of Android in the phone).
With hundreds of possible deals, someone else may spot something better, but knowing the phone is capable of using the net, displaying maps and so on, using Android, felt it might be worth pointing at a contract which (overall) should be within budget.
Of course it depends on how well Vodafone works in your area, too... I'm using ASDA PAYG (which uses the Vodafone network) and have no modern 'smartphone', so just a bit green with envy at people getting these nice gadgets!