Is the Bolton sigil meant to look like the British Flag?

The UK flag, the Union Jack, is a combination of three flags:

And Scotland's flag is based on the crucifixion of Saint Andrew, which was held on an X-shaped cross:

But Bolton's flag represents glorification of the cruelty of skinning people alive on an X-shaped cross upside down.

So they do have a similar basis of crucifixion on an X-shaped cross, but with a different meaning/intention, and I think it's just a coincidence and not intentional.

Curiously, both the Bolton banner and the Union Jack do have some common elements in how they came to look the way they do!

• The Bolton flag is, of course, an image of a flayed man being crucified:

  

• The diagonal part of the Union Jack comes from the saltire-shaped flags of Scotland and Ireland:

  

  And St. Andrew's flag of Scotland is cross-shaped because it represents the martyrdom of the Christian saint Andrew, who was crucified! From Wikipedia:

  According to legend, the Christian apostle and martyr Saint Andrew, the patron saint of Scotland, was crucified on an X-shaped cross at Patras, (Patrae), in Achaea. Use of the familiar iconography of his martyrdom, showing the apostle bound to an X-shaped cross, first appears in the Kingdom of Scotland in 1180 during the reign of William I. [...] Using a simplified symbol which does not depict St. Andrew's image, the saltire or crux decussata, (from the Latin crux, 'cross', and decussis, 'having the shape of the Roman numeral X'), began in the late 14th century.

So they're both X-shaped because of crucifixions, although the Bolton flag celebrates their own crucifixion of their enemies while the Union Jack and Scottish flag commemorate the crucifixion of their patron saint.

Additionally, the Union Jack is sometimes known as the butcher's apron, a pejorative term used particularly in Ireland - and the Boltons would certainly qualify as butchers.

I reckon so. The Union Jack may have its own history, but I doubt the similarity escaped the "Game of Thrones" designers, and I wouldn't be surprised if their version of the Bolton banner was intentionally made to resemble the British flag. George Martin's description of the Bolton sigil is very different, after all. Additionally, the wars to "unite" (or subjugate, take your pick) the British Isles involved a great deal of cruelty, including flaying of prisoners. Look up Hugh de Cressingham, for example.

I totally agree that the Bolton banner is intended for us to see a resemblance to real life flags such as the Union Jack. It's part of why I like the show so much and I'm pretty sure those who produced Song of Ice&Fire/Game of Thrones know what they're doing by this symbolism. In my opinion, it's what reminds us of ancient stories from times that there might have been real magic, real dragons, true evil and true good. When you consider that scenes like the "red wedding" are drawn from similar real life events, it's really not that much of a stretch to figure that they would make other symbolic parallels.