Under £30 Barlow Lens.

[Synq]

Looking for a under £30 Barlow Lens for my Heritage 130P scope. Recommend some please. Thanks.

[Louis D]

The Revelation/GSO Astro 2.5x Barlow tends to get good reviews here and on CN.  I've not used one since I'm quite satisfied with my vintage TV 2x, Meade 140, and Orion Deluxe Barlows.

[Zermelo]

If you could stretch to £40 you would have a bit more choice.

The Revelation 2.5x has already been mentioned, but it's usually £35- £40.
There is also The Baader Classic Q for a similar price.

The  Celestron Omni 2x is decent but RRP is £60. However it appears to have been on sale here for £29.95, when there was stock.
You could also consider https://www.365astronomy.com/GSO-2x-Barlow-2-Element-Achromatic-Barlow.html,  which is another GSO at £22.40, but a 2-element low dispersion design rather than the 3-element "semi apochromat" above.

Availability is, of course, restricted at the moment due to the effects of Covid.

[Second Time Around]

I've just bought a 130mm f/5 Dob for a friend as an Xmas present for her daughter.

I'd very much recommend a dual 1.5x/2x model as this gives much more flexibility.  Not all 2x Barlows allow this, but the ones that do so don't cost any more.  These dual models are sold as 2x Barlows, but some 2x models can also be used at 1.5x.  These ones allow the black lens cell to be unscrewed from the body of the Barlow and then screwed into the filter thread at the bottom of an eyepiece.  Very often this won't be in the blurb, but on their website the US retailer Agena Astro states whether this can be done or not under the specifications.  I'd also add that the 2x Orion Shorty Barlow that Agena doesn't sell can also be used at 1.5x.  The amplification factors are only approximate as it varies with the eyepiece used, depending where the focal plane of the eyepiece is located.

I'd use the 2x option on your scope on nights when the atmosphere is steady.  However, in the UK we're frequently under the jetstream, which means that the atmosphere is then more unsteady.  Then the 1.5x option on the Barlow would be a better choice. 

Most if not all of these Barlows are identical except for the name.  The forum sponsor, First light Optics, does an excellent one in their Astro Essentials range that even has a T thread at the top for attaching a camera.  The cost is only £25.  Go to https://www.firstlightoptics.com/barlows/astro-essentials-125-2x-barlow-with-t-thread.html.