With only one child, a nanny is an expensive option. Given the timings you have given, the 18:30 finish work time could be a problem. Is that the time you leave work, or the time you get home? What about the start time, is that work start time, or when you would be leaving home?
List of Nurseries and Childminders in Richmond, Surrey - this list is NOT complete, but will give you some idea of what childcare facilities are in the area.
A childminder in your area I expect will cost £6-£8 per hour. A nursery I expect will cost £60-£70 per day.
How old is your DD?
Do you want a home from home environment, or do you want more of a large group environment?
Do you want your DD to be with lots of children their own age, or with children of varying ages?
I will start off the Disadvantages/Advantages of the various types of childcare. Others will add to the list I hope.
Nanny
Cares for your child at your home.
Disadvantage: If they are ill, there is no backup.
Advantage: They are your employee so you have full control over what they do. They also do some domestic chores at home, such as trying to keep the kitchen tidy.
CM
Cares for your child at THEIR home.
Disadvantage: May be no backup if the CM or CM's own children are ill, though some CM's have links with other local CM's who may be able to help out. The CM will be collecting older children from school, so your DD will go on trips to/from school - this can be educational, can be a social event.
Advantage: Cares for children of multiple ages, so is like your DD being part of a family with older/younger siblings. During the day while older children are at school, your DD gets 1:3 / 1:2 attention as the CM would have your DD plus one or two children.
A CM will take your child out to local parks and other childhood attractions.
Nursery
Cares for your child at their location within a large group environment.
Disadvantages: Illness spreads quickly within large group environments, so when your DD starts, expect her to pick up illnesses on a regular basis. She will soon build up immunity though, so you may see that being an advantage.
You have no control over what happens, the nursery dictates everything. Some nurseries may try to follow routines your DD has, the nursery will have it's own routines - so your DD may need to adjust... such as sleep/nap times. Some nurseries do not take children out and about - your DD may be stuck inside the same four walls most of the day. On the other hand, some nurserys do take children out, for local walks to the park (being Richmond area, I would hope that nurseries would take children to Richmond park on occasion).
Advantages: If a staff member is ill, it is the nurseries problem not yours - this means that the nursery may get a temp in from an agency. You can collect when you like as children are often on nursery grounds the vast majority of the time, whereas with a CM the CM may be out and about quite a bit.
Your DD will be in a group with other children her own age - you may see this as being an advantage or a disadvantage.