Is the letter from the sponsor enough?
No, a sponsor letter is not enough, excluding the other needed evidence can be a disastrous strategy.
There are five (5) criteria used to determine if a person qualifies for a Standard Visitor Visa. From the rules, these are...
- (a) will leave the UK at the end of their visit; and
- (b) will not live in the UK for extended periods through frequent or
successive visits, or make the UK their main home; and
- (c) is genuinely seeking entry for a purpose that is permitted by the
visitor routes (these are listed in Appendices 3, 4 and 5); and
- (d) will not undertake any prohibited activities set out in V 4.5 – V
- (e) must have sufficient funds to cover all reasonable costs in
relation to their visit without working or accessing public funds.
This includes the cost of the return or onward journey, any costs
relating to dependants, and the cost of planned activities such as
private medical treatment.
Having a sponsor letter takes care of the last one, item (e), and that's only if they accredit your sponsor with having the capacity for sponsorship.
For the other criteria they use evidence submitted by the applicant to determine what the risks are. In the absence of satisfactory evidence, they will use one of their standard refusal formulae, like
...I acknowledge your sponsor proposes to pay for your visit, however,
it is the circumstances of the applicant that remain paramount when
assessing your application...
or this one...
...however whilst I take that into account in assessing your proposed
maintenance and accommodation in the UK, that is only one aspect of
the visitor rules and this sponsorship does not satisfy me of your own
intention to leave the UK on completion of your visit...
There are several more points raised in Should I submit bank statements when applying for a UK Visa? What do they say about me? that will be of interest to you.
What other documents need to be included?
The only required documents are a passport and the fee. Everything else is optional, but they publish a so-called "Visitor: supporting documents guide" which people use as a reference point.
...my sponsor has done the undertaking form letter
Normally if they want an undertaking they will tell you once they have looked at all your stuff. Also, they prefer using those when the applicant and sponsor are immediate family members. But based upon what you have written I doubt it will get that far. There's more about sponsorship issues in this article also: UK visa refusal on V 4.2 a + c (and sometimes 'e')