If you are indeed deficient, and it is correct that the test you get from you GP will be more or less useless, pills or sublingual are not really going to help.
Ideally, you'd want to do the other tests that give better indication of b12 deficiency, for a start doing the active test. You can buy one privately if you'd rather avoid your GP.
You need to take no supplements for reliable results for these, so once you start, it's too late (you have to be off them for at least 4 months).
Saying that, you can start injecting without a diagnosis as ultimately that will be the treatment if you are deficient and taking b12 will do no harm if you are not.
You can indeed buy off Amazon or through online pharmacies who export to the UK. The main one had stop shipping to the UK but I've heard has started again. Be careful not to buy in bulk though as now with Brexit, you'll have to pay tax and it's a lot.
The pernicious anemia Facebook group will give you all the detail of where to go to order.
You can order syringes and needles via Amazon and watch YouTube to learn how to self inject. It's really not hard. I've done it for 4 years now. No official diagnosis as my levels were high too and by the time GP was prepared to do more test, I'd already started to self inject. It changed my life.