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The refurb X-series machines are ace. They're relatively thick and heavy by modern standards, but they're also fast, durable and easy to repair or upgrade. The battery life isn't great compared to a modern ultraportable or Chromebook, but performance is excellent. Avoid the X201, as you can find an X220 for about the same price or an X230 for a little more.
The battery might be very tired in some of these machines, so budget for a replacement. It's also well worth considering an SSD upgrade, as it'll shave the boot time down to a few seconds and make the machine feel much more responsive. The X220 and above have an mSATA slot, so you can fit both a hard drive and an SSD if desired. A small mSATA SSD can cost as little as £20 on eBay.
The only reasonable alternative in that price range would be a cheap ultraportable like the Asus E200HA. These machines are just barely adequate in performance, but they're extremely lightweight and achieve phenomenal battery life. They're basically a Windows tablet with a keyboard bolted on, for better and for worse. Fine for basic browsing and office tasks, but they'll bog down quickly if you ask too much of them. You're looking at about £180 new or £130 b-stock.