Will I be classed racist if I have a confederate flag as a curtain, in my window in the uk?

Confederate flags are associated with several things.

1. Racism 

2. Pride in your southern heritage

3. Rebelliousness

4. Discontent with the government.

If you use a Confederate flag for a window curtain you can expect to raise some eyebrows. You will need to explain your intentions. Some people will assume you are a racist.

Possibly. What message are you trying to convey by displaying the flag?

You might be but it's not illegal so why care?

No, a flag is just a flag. It's just that some people associate a confederate flag with racism. Me personally I associate the confederation flag with the 80s TV show The Dukes of Hazzard. 

Yes, because it is considered a racist flag, and you will proberly get your windows broken.

The flag commonly known in the US as the Confederate flag bears a resemblance to the Union Jack which is the official flag of the UK. Many people in the UK, if they even noticed, would think it was an outdated version of the Union Jack.

Any that know US history enough to recognize the flag would know it IS NOT racist.

Very few people here would know or care what it is.

UK

No, but sort of a weird duck...  Since you aren't in the US, why would you even DO this?

No, just stupid.

If anyone recognizes it, probably.