The reason is
That £10 voice costs them 50 times less than £5 data.
Voice is about 12kbps symmetrical and in a perfect scenario what the pay out for out going calls termination charges is nearly balanced for what they receive in incoming call termination charges.
Many voice calls are on their own network.
Almost all Internet data is I/O on their network to others. All of that they pay for in both directions per MByte, not time.
So maybe 50:1 to 200:1 difference in profit for same Voice and Data Revenue
People want > 1Mbps for Data.
A Cell phone mast sector can only support maybe 3 to 5 people simultaneously streaming YouTube. So capacity/Congestion is a huge issue for data versus voice.
Also if people using it instead of Fixed Broadband, instead of a complementary "on the go" Mobile solution, their data sessions are very much longer than typical phone calls.
Another problem also is that for equal SMS and Data revenue, the profits are maybe 1000:1 as SMS costs them nearly nothing. Especially on their own network.