I think you need to reframe the way you value yourself, and your health.
My perception, from what you've said, is that your could theoretically afford a private appt. This is more important than Christmas.
Short term, If you end up spending Christmas day in bed you'll be no use to anyone. Long term, if it is hyperparathyroidism, you could end up with some significant health events which can be life altering.
(Btw magnesium levels are also related to parathyroid.)
I think if you had anaemia, that would've shown up at some point in the past few years. You can, at a very basic level, see if your gums, and inside lower eyelids, look relatively paler than you'd expect. Or compare to a family member of similar colouring.
If it was "just" about what you're eating, you would not be permanently exhausted.
If you find the website I suggested too overwhelming, you could ask the doctor if they think it would be sensible to check the following:
Full Blood Count (FBC)
UEC (Kidney function tests)
LFT (Liver function tests)
CMP (Comprehensive Metabolic Panel - looks at things that affect fluids, sodium, kidneys etc)
Ionised Calcium
Corrected Calcium
PTH (Parathyroid marker)
Vit D
Iron Studies
Thyroid panel (TSH, fT3, fT4, thyroid antibodies)
If you've also been putting on weight at a rapid pace, they should check cholesterol etc, and HbA1c, which gives a long term view of your blood sugar levels. Cholesterol, blood sugar, and blood pressure can also be raised (sometimes dramatically). With HPT.
If you do end up having any markets that indicate a mineral or hyperparathyroidism issue, they should order a dexascan (bone density scan, cheap and quick), and ultimately a sestamibi nuclear imaging scan (expensive, complicated, only shows up 70% of parathyroid tumours). Before you get to the scans though, you need a seriously good parathyroid/thyroid surgeon. Someone who does many a year. Not a general surgeon, not an ENT surgeon, and definitely not an endocrinologist. Whether you're paying or not, an endocrinologist is a waste of time and money at that point. They will act purely as a gatekeeper, to accessing the surgeon, which either they'll be happy to do, in which case your GP could've done that, or they'll want to check on you over a few months, which means more tests more waiting (and possibly more money if you're seeing them privately). The ONLY solution to primary hyperparathyroidism is surgery. Anything else is faffing.
BUT. PLEASE don't go off and order all these yourself. I hesitated in listing them because you may want to do that, but seriously, save your money for the doctor visit. I'm not saying don't spend money, I'm saying spend it where it counts. I've no idea how your system works. It maybe that you should order all of these, rock up to your GP with the results, and if they indicate a parathyroid problem say "please refer me to this parathyroid surgeon".
But what if they don't indicate parathyroid?
What if the GP wants to do them again?
How much would that many tests costs?
Are the private tests done in the same land as the public tests?
Are they following the same best practice as the labs your doctor would use?
I'm trying to balance arming you with the correct information, and giving you carte blanche. Having read the pp though, she's right, you're an adult, and ultimately it's your life and health.