The farm is a bit of a trek from London and it's not particularly big. I think you just need 2 hours if you take your time walking around, have a look at the gift shop and have a snack in the cafe.
Having said that, it's a beautiful place. About half the area is very bright at the moment whereas the other half is duller - I don't know if that's a different type of lavender or if it isn't in full bloom yet. I loved how the lavender changed colour depending on the sun/light - it looked very different from one minute to the next. It is a bit trampled in places. I guess people can't always be bothered to walk round a row which is a shame.
There're definitely some great photo opportunities but there're a lot of people about so it's quite difficult to find your own spot and you have to queue for the phone booth.
I had a hot dog in the cafe which I'm sorry to say was pretty poor but the lavender lemonade and ice-cream were lovely. The gift shop is nice. It's only £2.50 to enter the farm and you can redeem that £2.50 at the gift shop anyway - I didn't realise until I got home as I just pocketed the ticket immediately without looking at it.
The staff are friendly. There's a tractor ride for kids, a flower garden and beehives. I hate Portaloos like most people but I understand the reason for them.
I think if you're willing to make the journey it's definitely worth going to. Just be realistic about how many people you're going to encounter once you get there and how relaxing/peaceful you're going to find that. For me personally it was worth both the journey and the crowd and I had a good time.

I visited the Mayfield Lavender Farm at Surrey Downs from London. The “Southern Train” service is frequent from London Victoria station to Sutton or Cheam. Sutton is the 1st stop followed by Cheam. The quickies route to reach the farm is to take cab/minicab from the station. The entry fee is £2.50 per person. On arrival, you will be mesmerized by 25 Acres of bright purple blossoms. Purple horizon is sight to behold. There is a nice café with seating overlooking the field. The food consists of sandwiches, waffles, ice cream and you can purchase Lavender biscuits and shortcakes from the Gift Shop. The heart shaped Lavender biscuits (SEE photo) were well prepared and tasty. The Lavender white chocolate block was creamy with a hint of delightful Lavender (SEE photo). You will find the regular Lavender infused body care products such as creams and soaps available. Make sure you get the Lavender ice cream before leaving the farm (SEE photo). By the way, there is a 15 minutes Truck ride around the field, with Jordan (SEE photo) for £2 per ride. This place was dreamy and delightfully beautiful.

SEE photos.