Forget it. A booster would have to be an active device which could transmit and receive of the same frequency - it can be done but it's expensive.
The easiest solution is to get a PAYG phone from Carphone Warehouse and a couple of SIMS for services other than your own. If you have O2 get Vodafone and Orange SIMs, then switch on the Orange/T-Mobile roaming (on line.). A cheap phone wil cost you probably a tenner or less with a tenner top-up. SIMs are usually provided free from the service providers own shops subject to a tenner top-up as well. Packaging for O2, Voda, and T-Mob will show the number on the ouside, but Orange only set the number when you register. You can also request SIMs (and you then don't have to top-up at purchase) on line. Most SIMs (except O2) come with a small amount of credit which will allow the phone to access the system. You can then check it where you are resident first before putting money on it. Top-up can be done at any shop or petrol station which has the yellow Paypoint sign, and you can also top-up from many ATMs - usually those NOT operated by the big banks, i.e. Co-op, Alliance and Leicester, etc, and even on the phone itself using a credit or debit card.
O2 and Vodafone in rural areas tend to use 900MHz which works better over wider areas. Orange/T-Mob don't have that option and are stuck on 1800MHz which is more lossy. The latter also applies even moreso to 3. In particular O2 have traditionally the best rural coverage.
Just remember you need to have a chargeable event on a PAYG periodically - maybe once in three or once in six months - else you will loose your credit and number. Cheapest is to send yourself a one-word text (usually 12p.)
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