Xiaomi - Help with frequency bands and purchase

[ScratchCat]

I live in the UK and use the Carrier GiffGaff, a subsiduary of O2. The issue is that I plan on purchacing a new phone and would like to use 4G/LTE. The frequency band O2 uses is 800 MHz though which two of the phones in consideration do not support.

http://www.gsmarena.com/compare.php3?idPhone1=8291&idPhone2=8263&idPhone3=4664

http://www.4g.co.uk/4g-frequencies-uk-need-know/

Phones in consideration:

Xiaomi Redmi Note 4

Xiaomi Redmi 4 Prime

Elephone P9000

 

The question is: Even though the phones don't support the Band 20 , can it connect to other carriers cell towers using a different band and therefore enable me to use 4G?

Or would I only be able to use 3G?

If you have other recommenations or advice please do post your opinion.

Thank you

 

[Agosto]

Man, that's sad. O2 uses only one band and the slowest one? meh

I don't think you will be able to connect to other carrier towers normally, isn't it considered roaming?

[ScratchCat]

Ah I see, so would it cost money to roam in your own country?

I will probably go either with the redmi note 3 interational , the elephone, zuk z2 or SD redmi note 4

Thank you

[Agosto]

8 hours ago, ScratchCat said:

Ah I see, so would it cost money to roam in your own country?

I will probably go either with the redmi note 3 interational , the elephone, zuk z2 or SD redmi note 4

Thank you

I really don't know it shouldn't though 

 

What about the P9 Lite?