I hold a driving license from the outside of the UK which is not valid to exchange. At the same time, I already have an UK privisional license. Does it mean my foreigner license is not valid anymore? (I'm here less than 12 months)
I wanted to hire a car while I'm still in the process of taking theory and practical testes for the full UK license
Your non-UK license remains valid, and is not voided by the issuance of a UK provisional license.
You can drive a small vehicle (motorcycle or car) in the UK for up to 12 months using your non-exchangeable non-UK license. See the presentation on this UK govt webpage.
Whether you can hire a car in the UK using your non-UK license will depend upon the class of license you possess, your age, and the rules set by the hire car firm. You will increase your chances of success by also having a valid International Driving Permit, issued in the same non-UK country that issued your driver's license. (The IDP is not a license, but is an official translation of the terms of a driver's license issued by your country, and is especially useful or required when the driver's license is in a non-English language, or uses a non-Latin system of writing.)
Until the 12 month is over you are allowed to drive using your foreign licence. You won't need to put on 'L' plates or similar as well. However if you want to keep on driving without restrictions you have to pass the driving test before the 12 months are over.
From the ask the police site:
While your full licence is valid, you can drive any type of small vehicle shown on your licence for up to 12 months from the time you started living in Great Britain. To continue to drive after this you must pass a GB driving test before the 12 month period ends. If you get a GB provisional licence during this period, you will not have to display 'L' plates or be supervised by a qualified driver, and you will be able to drive on motorways. If you do not pass a test within the 12 month period, you will not be allowed to drive as a full licence holder and the conditions of a GB provisional licence will apply. If you do not apply for a provisional licence within 12 months, you must stop driving and get a GB provisional licence – the conditions of a GB provisional licence will then apply
