You CANNOT work as a dietitian in the UK without first getting a pre-registration degree or having your international dietitian registration approved by the relevant UK bodies. The dietitian profession is regulated by a government body called the HCPC, and just like e.g. nursing, radiography, etc. you must have registration to call yourself by that title.
Anyone can call themselves a nutritionist, but you will not get work in the NHS dietitian departments as a nutritionist. Unfortunately for you, this is the law here, as a way of protecting patients. So you have 3 options as I see it:
1. Take a pre-registration degree course (safest option);
2. Try to see if your Pakistan registration can be transferred to the UK (possibly a complicated process with no guarantee of success);
3. Work as a nutritionist (you will only be able to work privately or for some companies, you will never work in an NHS dietitian service).
I dont think I'm allowed to post direct links on TSR, so if you put "Pakistan dietitian UK HCPC" into Google, you'll see a HCPC website story of someone in a similar position to you who got their Pakistani dietitian registration recognised in the UK. So it is possible
I don't know much about HCPC dietitian recognition, but hopefully you can find some more relevant information