I'm not sure what a "£70 usb thingy" is, but I bought a turntable with a USB output at low cost and found the quality to be satisfactory -- and even better when the output was cleaned up using the freeware Audacity program to remove surface noise.
You'd then still need appropriate hardware to take the S/PDIF digital output and use it to create a FLAC file.
The easiest solution (but possibly not up to your desired standards) is to take the output from a good analog turntable (and phono pre-amp) and feed it into the audio input of your desktop or laptop computer using any one of a number of available digital recording software programs to generate your FLAC file.