As others have said keep your job and do your HGV tests in your own time.
Once you have your licence perhaps try some weekend work through an agency (as long as it doesn’t affect your current job).
Easier said than done but certain times of year agencies are crying out for drivers, or so they let us believe.
Be prepared to do the legwork when you have your licence and you just may find a company willing to take on a new pass. First impressions count and the best first impression is face to face.
Life’s all about options give yourself as many as possible.
You have the primary qualification already in that ‘you have always wanted to be a driver’.
Good luck and keep us posted, you will also find lots of similar threads on here that will make an interesting read.
Or take a look at the main forum where there’s lots of experienced drivers discussing aspects of the job. Oh and ■■■■, lots and lots of ■■■■ 
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