I noticed that the Likebook Boyue Plus 7.8 is available on Amazon for £150. This is a previous generation device but you still get capable specifications: 16GB store, 1GB RAM, 300 PPI E-Ink Carta screen and a quad-core CORTEX-A9 processor. The device runs Android 4.2.2 and so most e-reader applications should work. I would try KOReader and Bookari Reader. Bookari Reader is particularly useful as there is an option to switch to an e-paper interface.
The current generation (Likebook Mars) has a bigger battery, 2GB RAM, a more powerful processor (an octa-core Freescale RK3365) and runs Android 6.0. The updated version of Android, more RAM and a bigger battery are all useful but an octa-core processor is over-kill for an e-reader!