The Affinity Photo radial blur filter can be found in the blur category of the filter menu
It does not come with a vast number of options : angle which controls the strength of the effect. You can also interactively control the origin point of the effect which by default starts in the center of the image but can be shifted by the mouse / pen while the panel is active. If you want the effect to be coming from the bottom left, just click the bottom left. Subtle shifts can be made by using a low setting. At 180 degree max the entire image will be averaged out and you will see circular streaks, basically it becomes a circular gradient / noise effect
The effect can be applied once but it can also be re-applied multiple times via the repeat feature at the top of the filter menu. The effect can also be recorded via the macro feature and re-applied multiple times. It can also be faded via the layer menu and fade, allowing for the introduction of blending modes such as difference / darken etc to be added in combo with the image.
You can use it with a layer, creating even more amazing designs as the effect can be used to extend the layer beyond its original bounds. Select a layer and apply the effect and set the angle to 20 or 30 (if set too high the layer virtually vanishes) and then change the origin point. Click apply. This works really well combined with other effects such as deform and mirror. As the effect has smudged the edges of the layer, the deform effects becomes even more like dragging a mist or wisps of paint or the appearance of ghostly imagery.
It can also be used with selection. If applied to a selection, only that part of the image is modified. You can create all kinds of selections and then apply it to that. You can also modify the marching ants by using the layer menu and edit selection command and once in there, you can modify that using it
You can display the channels panel, select the composite red and apply it to just the red channel, you can then select the green and apply a different origin and angle to that and then to the blue.
You can use it with pattern layers, select that layer and then go to the top left and make certain mirror is on, without the mirror you will end up seeing breaks in your image. Apply it to the design. You can also use the live layer option as well and connect that to the tile as well as have the effect on top of the pattern and applied to all layers below. You can also use the lock children feature to change the pattern without moving the effect.
After applying it, you ca then go the layer menu and fade command. You can then set the opacity at 0% and full on effect with the blending modes. If you set it to 100% then you will just get the full on effect but if you change the blending modes then nothing will change, you just get the spin effect. You can create all kinds of amazing designs by using the fade combined with the effect (or just use the effect on a layer and then blend with the blending modes in the layers panel.