I don't know what featherweights cost in the UK. In the U.S., they typically run about $300.
Reproduction featherweights are definitely not worth buying. All of the others are simply a matter of personal taste. Some people consider the white and tan machines to be more rare and valuable; however, others prefer the black machines because they have certain characteristics the others don't have.
Aside from being in running condition, the other factor that affects price greatly is cosmetic condition. A featherweight in bad cosmetic condition will sell for half the price of one in good cosmetic condition.
The featherweights with the removable table (model 222 instead of 221) are much more rare and fetch higher prices. One in good cosmetic condition can sell for $2,000 or more.
Is your Brother machine too heavy to move around easily?