Poundland don't sell microcontrollers in their raw form - but if they did, they would probably sell these.
The STM32F0 range of 32 bit ARM micrcontrollers are designed for low cost applications that don't need on-chip USB.
They offer a 48MHz ARM M0 core, 16 - 64kB of flash memory and 4 - 8kB of SRAM and a good rich mix of peripherals.
Up to 2 USARTS
Up to 2 SPI
Up to 2 I2C
12 bit ADC - 10 channels
Several timers - including quadrature decoder and PWM
On-Chip USB - STM32F042 range
They are available in several package sizes from a 20 pin TSSOP to 64 pin LQFP, family members are pin compatible for easy upgrade - and they start at under £1 in 1 off quantity. For greater volumes - the price drops to around 34p.
The 48 pin LQFP is no bigger on the pcb than an ATmega328P-AU, but provides 35 lines of I/O plus the crystal oscillator and a 32768Hz oscillator for a Real Time Clock. It provides two USARTS and 2 SPI interfaces which massively increase the communications capability over the ATmega328. The 12 bit ADC offers a 4 fold increase in resolution over the Arduino. If you choose the STM32F042 range - they even come with on-chip USB, and don't even need a crystal.
There has never been a better time to take the Arduino philosophy and reinvent it using a cheap 32bit microcontroller - offering about 10 times the computational throughput.
By implementing it on a breadboard friendly 40 pin DIL footprint - a bit like the ARMiGo. It can be powered through the mini B USB socket on the right, and programmed via STLink and FTDI using the connector on the left.