Yes you could. Although if you don't know what you are doing and configure incorrectly or have compatibility issues it may be more hassle than the £4.99 you would have saved.
If you can borrow a dongle off someone why would you not be able to use their SIM, buy a top-up for it or just pay them for using it? I.E. why would you need to purchase another SIM?
O2 Broadband Premium LLU
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