The two most important requirements for admission into an educational institution in the UK are:

1. A strong academic background
2. Good command over English Language
Note that universities in the UK operate with a large degree of autonomy, that they set their own academic entry requirements & have got the freedom to take their own decisions on individual cases. 

Academic Requirements for most common courses: 

Degree courses: 

Students normally enter degree courses at the age of 18 but they can be younger and many are older. For admission to a degree course, you will need two year of university degree from Pakistan, completion of foundation degree or three A-levels. The grades you need will depend on which subject you choose and which institution you want to study at. 

Postgraduate programs: 

For entry to taught postgraduate courses (master's degrees, postgraduate diplomas and postgraduate certificates), you will need an four year undergraduate degree from Pakistan or two year master degree on the basis of two year bachelor degree or equivalent qualification. For example, to study chemistry at master's level, you would need to have a master degree in chemistry from Pakistan. 

However, 'conversion' courses are designed to provide an intensive introduction to a subject. For these programs any British undergraduate degree is sufficient, even one in an unrelated area. For example, you might have studied history for your first degree, but could use a conversion course to prepare for a Master of Science (MSc) degree in information technology. 

Master of Business Administration (MBA) 

Such courses usually require a British undergraduate degree or equivalent qualification same as for other postgraduate programmes of UK. However most MBA courses may also require two or three years work experience. GMAT may also be required. 

For a Doctor of Philosophy (PhD) 

You will need a UK master's degree or equivalent normally like MPhil from Pakistan. If you don't have a master's degree, you may be able to register initially for a Master of Philosophy (MPhil) or Master of Research (MRes) course and then transfer to the PhD program later if your progress has been good. 

English Language requirements

If your first language is not English, you will need to provide evidence that you have a good standard of English language skills. This is because during your stay in the UK, you need to attend lectures, & seminars, study academic printed resources, produce some written works - everything in English. 

Academic IELTS or equivalent requirement for Master and Bachelor degree is 6.5 and 6.0 respectively with not less than 5.5 Band in any module. For foundation degree courses 5.5 can be acceptable. 

IELTS requirement can be waived at a UK University for gifted (exceptional) students but for this the Academic Registrar at the Higher Education Institution will need to personally authorise the issue of the CAS for the Tier 4 (General) student, which must confirm that the Academic Registrar has approved the waiving of the language requirement. 

From 21 April any Tier4 (General) student enrolling on courses below NQF level 6 and its equivalents, including those coming to the UK to follow English language courses, but excluding those on Foundation degrees and HNDs in Scotland, his/her Tier 4 sponsor must ensure that he/she is competent in English language at a minimum of CEFR level B1 (IELTS score of 4.0 or above in all modules) and for all course above NQF level 6 and equivalent minimum of CEFR level B2 (IELTS score 5.5 or above in all modules) is compulsory. 

Please find list of approved English language tests and equivalency information. 
http://www.ukba.homeoffice.gov.uk/sitecontent/applicationforms/new-approved-english-tests.pdf