I have been rejected for visas 5 times in 4 countries. 3 of those were Schengen countries. Now, I want to study in the UK. Will it be a big issue?. Will the UK home office know everything about my rejection?
The application form asks:
For either the UK or any other country, have you ever been:
- Refused a visa
- Refused entry at the border
- Refused permission to stay or remain
- Refused asylum
- Required to leave
- Excluded or banned from entry
It then asks for further details (what happened, what country, when, and free text details). It will ask for it for as many events as necessary.
So, as you will not lie about it, they will know.
If you think about lying about it, don't. There are good chances they will find out, and if they do, you'll be in a lot of trouble, and will not only be refused a visa, but will most probably be banned for many many years.
Having been refused so many times is definitely not in your favour, though we don't know the details about those rejections, when they happened, why they happened, and if your circumstances may have changed since then. Even if the reason for your visa request has changed, you should most definitely make sure you address all the issues they raised before applying for a UK visa.
Given the information contained in this answer in this SE:Travel question, I think the UK is extremely likely to know about your Schengen refusals.
You haven't named the non-Schengen refusal countries, so no one can answer as to them.
As mentioned in this SE:Travel answer, you are required to provide such information. If not provided you will almost certainly be refused for deception.
As it is, your case isn't strong. 1 visa rejection isn't good, 5 is awful. You should be completely honest in explaining the refusals and the reasons for them. Schengen and UK do exchange information so most likely the home office will know.