Ever since I entered UK in Oct 4, there's no MTN signal in my phone to roam

Hi Ubong, you need to roam your line with any of the MTN preferred roaming partners in UK. You can roam with Vodafone or Telefonica O2.
Just power off and power on your phone, your phone will select any of the preferred available roaming networks. If your phone did not select the preferred roaming networks, you can do manual search depending on your phone type.

Manually search for the preferred network provider.
• From ‘Settings’, click ‘More’
• Select ‘Mobile network’
• Select ‘Network operators’
• Select MTN SIM(either SIM1 or SIM2 as applicable)
• Click on ‘Search network’
• Select the preferred network provider from the list displayed on the device.

If the issue persist, you need to contact MTN support for assistant.
You can contact MTN support via any of these channels:
1. MyMTN App
2. Facebook: @MTNLoaded
3. Instagram: @MTNNG
4. Live Chat: http://bit.ly/2OC5bHz
5. Email: customercareng@mtn.com
6 .WhatsApp: 09033000001