The question is usually asked with a fee in mind, so it is worth separating two different things: the work an agent does for you, and the duties the law puts on them whether they want them or not. The first is what you are paying for and varies enormously between agents. The second is your protection and does not vary at all.
The work: valuation, marketing, viewings, negotiation
An agent prices the house, prepares the listing and the photography, puts it in front of buyers, handles enquiries and viewings, and negotiates offers. After acceptance they chase the chain, which is the least visible and often the most valuable part of the job. How much of this the agent does themselves rather than leaving to you is the real difference between a high street quote and a low online one, and it is the thing to pin down before you compare rates.
The duties: what they must tell you, whatever the fee
Before you enter the contract, an agent must give you the circumstances in which you will become liable to pay and the amount of their remuneration. Once instructed, they are obliged to pass on every offer for your home right up to exchange of contracts, not only the ones they think you should take. And if they or a lender recommend a conveyancer, broker or surveyor, you must be told if they receive a referral fee for it, and you are free to use someone else.
Where to complain when they do not
Every agent belongs to a redress scheme, which is either The Property Ombudsman or the Property Redress Scheme, and you can ask yours which. You follow the agent's own complaints procedure first, and if they do not respond or you are unhappy with the response, the relevant ombudsman investigates for free. Knowing this before you instruct is worth more than it sounds, because it tells you that the duties above are enforceable rather than advisory.
Questions people ask about what do estate agents do
Does a higher fee mean more work?
Not reliably. It usually means more of the work is done for you rather than by you, which is a different question and the one worth asking directly.
Must the agent tell me about every offer?
Yes, right up to exchange of contracts, including ones they consider too low.
Do I have to use the conveyancer they recommend?
No. You must be told if they are paid a referral fee, and you are free to instruct whoever you like.