First of all define your scope of activity. A proper risk assessment provides points of reference. Will you be digging around in hedges, finding syringes, broken glass etc.
Next list the equipment you will need. bags, hoops, litter picking sticks, ties, stout shoes and ppe. Any hazardous waste will have to be approached using different criteria.
Work out the area to be cleaned and guestimate a working time, bags per job, petrol etc..
Ensure you have an adequate disposal method, preferably get the client to remove the bags, otherwise you will need a waste carriage licence and then a waste audit trail and pay for the cost of tipping.
Factor the above in and add external labour at a minimum of £15 per hour (or £3 above the agency rate just to break even).
We have just dumped a job for a large supermarket chain doing this in their car parks. The person we worked for sub contracted us paid for the first three jobs, then went quiet and now owes us £4k so be wary. We used around 30 bags per day and employed 3 people charging £20 per hour pp for an 8 hour day. The waste was placed in their bins so no disposal costs.
For a one man team including equipment costs but not disposal I would quote a minimum of £285 + vat.